Photovoltaic-Thermal Solar Collectors – A Rising Solar Technology for an Urban Sustainable Development

*Diogo O. Cabral*

#### **Abstract**

The increasing global warming awareness related to climate change due to the high emissions of carbon dioxide in recent decades linked all nations into a common cause, which requires ambitious efforts to combat climate change by adapting energy systems to its effects. This book chapter aims at investigating the potential role of Photovoltaic-Thermal (PVT) solar collector technologies for an urban sustainable development based on the current state-of-art, system components and subsidies for PVT technologies. PVT technologies are a practical solution to compete with isolated systems such as photovoltaic (PV) modules and solar thermal collectors if a significant reduction in manufacturing cost is achieved, coupled with an increased energy production performance. Therefore, its success is intensely linked to the capacity of the PVT industry/researchers to scale down its current system cost and complexity in a way that can shorten the cost/performance gap to both PV and Solar Thermal (ST) technologies. The knowledge gained presented in this book chapter has been acquired through an extensive literature review, market surveys and project development made by several PVT experts with extensive expertise in the development of PVT technologies, which establishes the foundations for more efficient and cost-effective PVT solar collectors.

**Keywords:** photovoltaic-thermal collector, PVT system assessment, performance evaluation, urban development, sustainable development

#### **1. Introduction**

#### **1.1 Fundamentals of PVT collectors**

The quest to decarbonize electrical and solar thermal (ST) systems has never been more urgent. While decarbonisation of the electrical system is on track, the decarbonisation of ST systems has not been tackled. ST systems typically make up about 50% of the final energy demand and [1] suggests that a large portion of the demand could potentially (i.e., while requiring significant technology developments)

be supplied by renewable photovoltaic thermal (PVT) solutions. PVT solutions address another important and increasingly emerging issue—spatial and network constraints, thus requiring less space than a PV or ST collector would.

Solar energy systems are progressively increasing their installed capacity due to subsidies and incentives as well as due to their increased efficiency [2]. Higher efficiencies and economic competitiveness increase annually, which leads to more investment and a sustainable energy mix.

The active application of solar energy technologies relies mainly on the use of photovoltaic (PV) systems for electricity generation and ST systems for heat generation.

The electrical efficiency of PV cells is typically around 22% for multi-crystalline and 27% for mono-crystalline silicon wafer-based technology [3], which corresponds to a fraction of the incident solar radiation. The remaining share is converted into heat. Additionally, the highest lab efficiency for thin-film technologies, CIGS and CdTe, is 23% and 21%, respectively.

The higher combined thermal and electrical efficiencies (per unit area) of a PVT solar system have the potential to overcome the typical low surface power and energy density of PV modules, and the relatively low exergy of the ST solar collectors, as well as the limited available roof/ground area [2]. This is particularly important if the available roof area is limited, but integrated solar energy concepts are needed to achieve a climateneutral energy supply for consumers, such as in residential and commercial buildings.

By co-generating both heat and electricity from the same gross area, PVT collectors extract the excess thermal energy generated by the PV cells by employing a heat transfer cooling fluid (HTF), increasing electrical efficiencies due to lower PV cell temperatures.

For solar energy applications, the wavelengths of importance for solar energy applications are typically from around 0.3 to approximately 2.4 μm of the solar spectrum (i.e., ultraviolet, visible and infrared region). PV cells optimally operate at a narrower range of the solar spectrum (i.e., from around 0.3 to approximately 1.1 μm), therefore the radiation that is not within this range merely warms the PV cells and can be used as thermal energy, thus limiting the maximum electrical efficiency [2].

Due to the co-generation of heat and electricity, PVT collectors utilise a broader solar irradiance spectrum, which makes them more attractive in terms of energy conversion effectiveness as can be seen in **Figure 1**.

#### **Figure 1.**

*Spectral distribution of solar irradiance, optical and heat losses, and heat and electricity gains. Provided by Manuel Lämmle.*

*Photovoltaic-Thermal Solar Collectors – A Rising Solar Technology for an Urban… DOI: http://dx.doi.org/10.5772/intechopen.104543*

**Figure 1** presents the wider range of spectral irradiance operation of PVT technologies, by employing both thermal absorbers and PV cells in the same solar collector box. As previously stated, PVT solar collectors are the combination between a PV module and an ST collector into a single unit. The PV elements convert the incoming solar energy into electricity, which is typically encapsulated with ethylene-vinyl acetate (EVA) or a solar silicone gel in case of low concentration PVT solar collectors [2].

On the other hand, the thermal elements of the PVT collector convert the solar energy into heat gains typically by absorption. The absorption is done at the receiver level, in which the harvested heat from the PV cells (highest material share exposed to the solar radiation) and PVT absorber (e.g., thermally couples the PV cells to the HTF) is transferred into an HTF. A schematic cross-section of a PVT collector is presented in the following **Figure 2**.

#### *1.1.1 Temperature dependence overview*

For a solar collector to produce usable thermal energy, the HTF must be at lower temperatures than the absorber (i.e., in solar thermal collectors) and PV cells (i.e., in PVT collector) as "heat can never pass from a colder to a warmer body without some other change" (statement by Clausius from 1854). Therefore, the thermal coupling between the PV cells (hottest element in a PVT collector) and the thermal absorber (and thus the HTF) is of most importance for the overall performance of a PVT collector.

Furthermore, silicon wafer-based PV technology is typically more efficient at lower module temperatures as their average temperature coefficient is around 0.35%/° C, which leads to a lower open-circuit voltage and thus a decreased electrical efficiency [2, 5].

Just as PV modules reach higher efficiencies at lower module temperatures, the solar thermal collectors do so too as the heat losses are proportional to the receiver's surface temperature. It is important to note that the operating temperature is regulated by the overall system (i.e., the temperature required at the heat storage) and not solely by the solar collector.

#### **Figure 2.**

*Representation of an uncovered PVT collector cross-section composed by a sheet-and-tube heat exchanger and back insulation: Anti-reflective cover; front-encapsulation layer (e.g., EVA); Back-encapsulation layer (e.g., EVA); Backsheet (e.g., PVF); PV cells; thermal absorber (e.g., aluminium, copper or polymers); thermal insulation (e.g., mineral wool, polyurethane) and frame. Based on [4].*

According to Lämmle [6], a solar thermal collector mean operating temperature typically ranges between 30 and 90°C. On the other hand, a PV cell typically operates at a module temperature between 30 and 60°C, which overlaps (to some extent) with a solar thermal collector mean operating temperature. This overlap in operating module and mean temperature leads to different behaviours from the PV module and the ST collector, as the ST collector has a deeper decrease in efficiency than the PV module for increasing temperatures.

Moreover, the PVT collectors do not operate at optimum efficiency for either PV or ST operation mode, which leads to a compromise between both elements. Therefore, PVT collectors operate either as *electricity* or *as heat optimum operation*, which prioritises the operating agent needs for a specific application, either giving priority to the electricity or heat generation [2]:


For an overall PVT collector optimum performance, it is crucial to efficiently utilise the available solar resource, therefore, it is of greatest interest to instal PVT solar collectors for better use of space.

Amongst several solar specialists, [1] consider PVT technology more complex than the available mature technologies such as PV or ST technologies. Nevertheless, PVT provides significant advantages such as the ones mentioned below.


*Photovoltaic-Thermal Solar Collectors – A Rising Solar Technology for an Urban… DOI: http://dx.doi.org/10.5772/intechopen.104543*

*HP that will make good use of the stored energy. The HP enables higher output temperatures enabling more compact storage solutions to be implemented. This is critical when space is limited. Larger thermal energy solutions can be accessed via district heating (DH) networks and enable the produced summer excess heat to be stored seasonally for the winter'.*


The relatively small emerging solar markets and small-scale production gives higher costs in the beginning, compared to the well-developed markets for fossil fuels. This cost disadvantage has all the time been the main barrier for both Solar PV and Solar Thermal.

A very important sometimes forgotten barrier is also proven long enough lifetimes for the new solar technologies. Many of the first concepts have failed in small things, as almost all new products, giving a bad reputation and extra costs for repair when introduced with too little product testing. For fossil energy supply the hardware stands for a much smaller fraction of the total cost and can be repaired at a lower cost per produced kWh when there is a problem. In this reliability respect, PV has had a great advantage, as it was first developed for Space applications, where the durability and reliability requirement is extreme. So, when "coming down to earth" that barrier was already solved. Only the cost was a barrier that could slowly be solved, by larger and larger markets and thereby mass production. Solar thermal has not had that kind of well-paying niche markets to the same extent and in almost all countries the subsidy systems have been an insufficient and too short term to develop a sustainable market.

For PV and solar thermal there were also niche markets in remote places without an electric grid, or for solar thermal replacing wood and oil during summer for hot water heating with low efficiency of the burners in these periods. Heat pumps have then become much more reliable in parallel and lower in cost and become a hard competitor for solar thermal (and can also compete with oil/fossil fuels). In a heat pump system, the PVT can find a niche market as one example, as it produces both heat for the cold side of the heat pump and electricity for the heat pump compressor operation. The cold side heat supply is sometimes forgotten when thinking about heat pump solutions. The heat pump still needs heat, its function is to increase the temperature of the available low temperature "free" heat to a useful level. This heat has to be inexpensive to make the whole system cost-effective. Here the PVT heat can make a nice contribution.

A further barrier for PV and solar thermal especially at higher latitudes is the annual distribution mismatch of demand versus energy production in many applications. This mismatch in demand and renewable supply is partly driven by the lack of solar radiation, causing lower outdoor temperatures and higher load in winter. In larger systems, seasonal storage in water pits can be used but in small systems, the heat losses in small thermal storage are too large for long-term storage. Phase change materials might be used then.

For PVT there is a further barrier that there has to be a reasonable match between supply and demand for both electricity and heat, to have full success. Oversizing gives longer payback times. Often too much heat is produced compared to electricity for the demand in a house, so efficient electric appliances can be extra cost-effective then. In systems where there is a need for heating a swimming pool or a borehole heat pump, the inclusion of PVT technologies could be a wise solution. Ideally, a total system view should be used when looking at PVT systems.

However, from these barriers can be concluded that several system types suit PVT technologies. It has been classified by the heating/cooling demand, as the electricity always can be consumed instantaneously or exported if too much power is produced.


#### **1.2 PVT collector classification**

According to Zondag [7], PVT collectors can be classified into four main categories according to their heat transfer medium, employed PV cell technology, collector design and their specific operating temperature.

Typically, PVT solar collectors either have air or liquid as an HTF, the latter being either water or a mixture between water and glycol (anti-freeze and anti-corrosion product) [8]. PVT air collectors are less sensitive to overheating as typically they are categorised as unglazed PVT collectors (i.e., glass cover, thus higher heat losses). On the other hand, PVT liquid collectors comprise the biggest installation share; however, they present overheating issues. Nevertheless, water (as a heating fluid) has a higher heat capacity and thermal conductivity than air [7].

The exponential increment in the efficiency of PV cells in recent decades tends to raise end users' expectations. Therefore, the system where this technology is employed is of most importance, since the specific suitability depends on electrical conversion efficiency, temperature and absorption coefficient [9]. c-Si PV cells have the highest share for both PVT and PV collectors. Mono-crystalline cells have enhanced electrical efficiency and solar absorption than polycrystalline PV cells. Thinfilm technologies, which comprise CIGS and CdTe technologies, are typically characterised by their lower temperature coefficient than c-Si PV cells, thus more suitable to work under higher HTF and module temperatures. In applications such as

*Photovoltaic-Thermal Solar Collectors – A Rising Solar Technology for an Urban… DOI: http://dx.doi.org/10.5772/intechopen.104543*

PVT collector, where cooling is needed at PV cell level, multi-junction (IIIV cells) solar cells are generally used for systems where high concentration is required. Therefore, this technology (IIIV cells) can be a contender for PVT solar collectors that require higher operating HTF temperatures.

Additionally, PVT collectors can be classified into two main clusters according to their design, such as flat plate and concentrating PVT collectors. Flat-plate PVT collectors can be sub-categorised into unglazed (e.g., similar aesthetics to PV modules, but without front glass cover) and glazed (e.g., similar in design to PV modules, with an additional front glass cover to decrease heat loss) PVT collectors.

Moreover, concentrating PVT collectors can be labelled due to their concentration ratio, such as low, medium and high concentration factors. Typically, low concentration PVT collectors are used as stationary (fixed collector tilt angle) solar systems. On the other hand, high concentration requires variable collector tilt angles and thus entails one or two-axis-tracking systems.

A balanced operating fluid temperature is critical to reach higher electrical and thermal efficiencies. Hence, the generality of the PVT water collectors and ST collectors can be allocated into three main groups for a wide range of applications [2]:


The temperatures of a specific system depend on the requirements of the heat supply system for DHW and space heating. Therefore, [9] allocated (in a schematic view, **Figure 3**) each PVT technology and applications per operating temperature.

**Figure 3.** *Map of PVT technologies and applications per operating temperature [9].*

#### **1.3 Performance of PVT collectors**

Solar radiation reaches the module at a solar irradiance, which immediately a fraction is lost to the ambient as *Qloss* and the remaining portion empowers the PV module (*Qel*) with a given electric efficiency (*ηel*). The accumulation of solar energy increases the temperature of the PV module and generates the thermal power of *Qth*, depending on the fluid medium and module design, which is transferred to the thermal module through a heat transfer mechanism with a thermal efficiency of *ηth*. Finally, thermal insulation obtained by reducing and eliminating the back and sides heat losses will increase the system efficiency. The general energy equation for a simple PVT module and overall efficiency (*ηPVT*) can be defined by Eqs. (1)–(3) [10, 11].

$$\eta\_{el} = \frac{Q\_{el}}{GA} \tag{1}$$

$$
\eta\_{th} = \frac{Q\_{th}}{G.A} \tag{2}
$$

$$
\eta\_{PVT} = \eta\_{el} + \eta\_{th} \tag{3}
$$

Where *G* (W/m<sup>2</sup> ) is the solar radiation and *A* (m<sup>2</sup> ) is the aperture area of the module.

#### *1.3.1 Electrical efficiency*

PVT systems are two separate systems that consist of a single ST collector and a PV module, which are attached together and work simultaneously to generate electricity and thermal energy. The performance of a PVT collector is reduced when the temperature of the system rises [12]. For a separate PV module, the following Eq. (4) provides the electrical efficiency *ηel*.

$$\eta\_{el} = \frac{I\_{mpp} \cdot V\_{mpp}}{G.A\_c} \tag{4}$$

*Impp* stands for the maximum power point current, *Vmpp* for the maximum power point voltage and *Ac* for the collector gross area in m<sup>2</sup> [13]. A special maximum power point tracking controller in the system assures that the PV modules operate at the best working point (*Impp*, *Vmpp*).

The reduction of the PV module performance with increasing temperature is given by Eq. (5), which represents the traditional linear expression for standard PV module electrical efficiency.

$$
\eta\_{el} = \eta\_{0,el} \cdot \left[1 - \beta \left(T\_c - T\_{ref}\right)\right] \tag{5}
$$

Where *Tc* is PV cell temperature,*Tref* is reference temperature and *β* is the coefficient of temperature. Nevertheless, by employing flash tests in which the PV module electrical output is measured at two different temperatures for a given solar radiation flux the above mentioned-parameters can be obtained. The real temperature coefficient value depends on both PV material and *Tref* [14–16].

*Photovoltaic-Thermal Solar Collectors – A Rising Solar Technology for an Urban… DOI: http://dx.doi.org/10.5772/intechopen.104543*

#### *1.3.2 Thermal efficiency*

Based on ISO 9806: 2017 at steady-state conditions for glazed liquid heating collectors, the instantaneous efficiency *ηth* shall be calculated by statistical curve fitting, using the least-squares method, to obtain an instantaneous efficiency curve of the form presented in Eq. (6).

$$\eta\_{th} = \eta\_{0,th} - a\_1 \frac{T\_m - T\_a}{G} - a\_2 \frac{\left(T\_m - T\_a\right)^2}{G} \tag{6}$$

where *Tm* is the mean temperature of heat transfer fluid (°C),*Ta* is ambient air temperature (°C), *η0\_th* is peak collector efficiency (*ηth* at Tm-Ta = 0°C), *G* is hemispherical irradiance, *a1* is heat loss coefficient (W/m<sup>2</sup> K) and the temperature dependence of the heat loss coefficient comes as *a2* (W/m2 �K2 ) [17].

#### **1.4 PVT systems: Types, components and applications**

Typically, a PVT collector operates in a solar thermal system, which affects the electrical and thermal yields substantially since its efficiency is temperaturedependent. *'The PVT system is amongst others characterized by its hydraulic layout, the sizing of storage and collector field, design temperatures of the heat supply system, and the system control'* [18].

It is crucial to create a context regarding the collector yield with its specific interaction between the collector, system components, weather, controller and user behaviour.

Lämmle et al. [18] selected four reference systems, which cover a wide range of promising applications and operating temperatures. A simplified hydraulic layout for each system with corresponding collector and storage dimensions is presented in **Figure 4**.

#### **Figure 4.**

*System diagrams: (a) solar heat pump system in parallel/regeneration configuration in a single-family house; (b): Domestic hot water in a multi-family home; (c) domestic hot water in a single-family house, system; and (d) combi system in a single-family house [18].*

System (a): A ground-coupled brine-water heat pump (HP) system incorporated into a single-family house (SFH) supplies space heat and DHW. By coupling a PVT collector to the cold side heat source of a HP or regeneration of a ground heat exchanger can potentially provide lower PVT collector temperatures and thus higher efficiencies.

System (b): A Domestic hot water (DHW) system in a multi-family house (MFH) is typically dimensioned to reach relatively low solar fraction. Therefore, the HTF is typically preheating and the overall operating collector loop temperatures are lower.

System (c): DHW system in a SFH is the classical system for solar thermal collectors and is therefore considered a promising application with a potentially big market for PVT collectors [7]. If the PVT system is not oversized compared to the load the operating temperatures can be quite low.

System (d): Combined DHW and space heating (combi) system in a SFH represent a challenge for PVT collectors due to the challenging thermal requirement efficiencies during winter, as the heat demand typically occurs with low levels of irradiance and ambient temperatures. Here avoiding oversizing is very important.

The electrical system can also be coupled with an electrical power meter, power optimizers in each PVT collector, battery storage systems and smart controllers optimising the interplay with the electricity grid.

Moreover, under the SHC-IEA Task 60 framework, a detailed representation scheme has been developed for combined electrical and thermal energy flows in PVT systems, which can be seen as an enhancement of the work developed at SHC-IEA Task 44.

In general, the system boundaries such as the final purchased energy, useful energy used in space heating, as well as system components (i.e., PVT collectors, heat pump and thermal storage) are represented and highlighted with explicit colours.

The system components are defined and highlighted as follows:


Furthermore, three different system boundaries were defined as 'left', 'right' and 'upper boundary':


*Photovoltaic-Thermal Solar Collectors – A Rising Solar Technology for an Urban… DOI: http://dx.doi.org/10.5772/intechopen.104543*

#### **Figure 5.**

*System "square view" connection diagram, which comprises the system components (highlighted left) and boundaries (highlighted right) [19].*

The scheme visualisation is very similar to other energy flow charts, yet it differentiates from previous ones as it has fixed boundaries, positions and colours, which are well defined by 'connection line styles'.

Within the system boundaries, different elements are highlighted (via placeholders) if they take part in the system layout/schematic. In case a specific component is not used, it is also shown in **Figure 5** without any highlight (i.e., no 'connection line styles').

The system components and boundaries have been differentiated by colours, such as Orange for Energy Converters, Blue for Thermal storages, Light Blue (colour also used for electrical energy flows) for Electrical storages, Grey for Final Energy, Green for Environmental Energy and Red for Useful Energy.

The system components are connected by arrows/lines, which represent the system energy flow. As shown in **Figure 5**, six different line styles are used for the indication of:


### **1.5 Current state of PVT technology<sup>1</sup>**

The past years showed that the PVT market is gaining momentum, especially in European countries, where the highest share of installed capacity of PVT collectors is located. Recently, the exponentially growing number of specialised PVT

<sup>1</sup> The data presented in this chapter has been acquired from the Solar Heat Worldwide report (2018, 2019 and 2020).

manufacturers that entered the European market, increased the awareness and interest in this technology, which led it to be included in the market survey developed by the Solar Heat Worldwide consortium. The report presented in both 2018 and 2019 included data from the work developed by experts in both PV and ST technologies, who are enthusiastic and share a common passion for this emerging solar technology. A market survey has been carried out under the works made by the IEA SHC Task 60 participants on "Application of PVT collectors."

By the beginning of 2019 (relative to 2018), a cornerstone had been reached of more than 1 million square meters of PVT collectors installed, in more than 25 countries.

The report developed by Weiss and Spörk-Dür [20] presents the global market developments and trends in 2019 of PVT solar collectors, in which the total area installed is around 1.167 <sup>10</sup><sup>6</sup> <sup>m</sup><sup>2</sup> (e.g., 675 <sup>10</sup><sup>3</sup> <sup>m</sup><sup>2</sup> in Europe, 281 <sup>10</sup><sup>3</sup> <sup>m</sup><sup>2</sup> in Asia, <sup>134</sup> <sup>10</sup><sup>3</sup> <sup>m</sup><sup>2</sup> in China and 70 <sup>10</sup><sup>3</sup> <sup>m</sup><sup>2</sup> for the rest of the world). Overall, it accounted for 606 MWth, 208 MWpeak of the total installed capacity, which was provided by 31 PVT collector manufacturers and PVT system suppliers from 12 different countries.

Within the countries with the highest capacity installed, France has to date around 42%, South Korea 24%, China around 11% and Germany with roughly 10%. The market for PVT collectors registered a significant global growth of +9% on average in 2018 and 2019. This trend was also observed in the European market with a slightly higher growth rate of +14%, which corresponds to an increase of the annually new installed thermal and electrical capacity of 41 MWth and 13 MWpeak, respectively [2].

Unglazed (also known as uncovered) water collectors are the most disseminated PVT technology with its largest market share of around 55% followed by air collectors (43%) and covered water collectors (2%). Evacuated tube collectors and low concentrator PVT play only a minor role in the total number of PVT installed capacity.

PVT technology suppliers commissioned at least 2800 new PVT systems worldwide in 2019. The number of PVT systems in operation at the end of 2019 was 25,823, of which 3296 uncovered PVT collectors were in operation, corresponding to a gross area of 667 <sup>10</sup><sup>3</sup> <sup>m</sup><sup>2</sup> . Out of these systems, solar air (pre)heating and cooling for buildings has almost 86% of the PVT installations, trailed by DHW for single-family households with 7% and finally followed by solar combi-systems (e.g., for DHW, space heating, multifamily houses, hotels, hospitals, swimming pools, and district heating) with just 7%.

In a global context, solar air systems (i.e., including PVT air collectors) have the highest share of the PVT market, with the majority of the installations being located in the French market.

#### **1.6 PVT collectors state of art**

Previously, PVT collectors have been classified according to their design, either as flat or concentrating PVT collectors, therefore, the literature overview in the following chapter is strongly focused on the current PVT solar collector state of art.

Glazed liquid-based HTF PVT collectors aim at replacing conventional solar thermal collectors given the similarity between systems and operating temperature range. Zondag [7] expects these types of PVT collectors to overcome the challenge of temperature stability reaching higher shares of the solar market. Moreover, Zondag [7] expected that researchers would focus on temperature-protected PVT collectors with overheating protection, which was the aim of the work developed by Lämmle [6].

*Photovoltaic-Thermal Solar Collectors – A Rising Solar Technology for an Urban… DOI: http://dx.doi.org/10.5772/intechopen.104543*

**Figure 6.**

*Comparison of the electrical and thermal efficiency of best of market unglazed, glazed, and low-concentrating PVT collectors. Efficiency related to aperture area (provided by Manuel Lämmle and presented in [6]).*

There are several commercially available concentrating PVT collectors, such as the stationary low concentrating PVT from Solarus and the line focusing PVT that was introduced by Absolicon AB and SunOyster Systems GmbH.

The motivation behind liquid-based concentrating PVT collectors is to replace the conventional thermal absorbers and at the same time decrease the amount of active PV area by applying cheaper reflectors. The reduced active PV cell area decreases the overall radiative heat losses due to the reduced hot surfaces. Stationary low concentration factor solar collectors (typically below 10 suns) do not reach temperatures higher than 120°C [9], as they do not need the use of tracking systems due to their relatively high acceptance angles [21].

Lämmle [6] made a direct comparison between ("the best PVT collectors in the market") unglazed (from MeyerBurger) and glazed (from EndeF) flat-plate PVT, low concentration (from Solarus) PVT, and a standard PV and ST module, to assess the current state of both thermal and electrical performance of the available PVT collectors. The results have been presented in the following **Figure 6**.

The thermal peak efficiencies range from 48% (for unglazed PVT) up to 53% (for the CPVT). These values fall below the 80% for standard flat plate solar thermal collectors due to simultaneous electrical generation (i.e., the fraction of incident solar radiation is directly converted into electricity), lower absorptance and higher emittance of the receivers (i.e., higher reflection losses), in most cases, a higher thermal resistance between the PV cells and the HTF.

On the other hand, both unglazed and glazed PVT collectors can compete with thin-film PV modules, but not with the high-efficiency mono-Si modules that reach around 22%.

Overall and as stated previously, a higher electrical efficiency leads to a lower thermal efficiency, which reinforces the educated "rule of thumb" that PVT collectors can either be optimised for high electrical or thermal performance.

#### **1.7 Needs for different key actors**

To establish a sustainable PVT system market there are needs for improvements on all levels. A market survey has been conducted to address the most relevant key factors for PVT technologies, where the following general needs can be pinned down as:


Moreover, different needs for Key Actors are required from a different set of intervenients such as:

*Researchers*: Development of standards and planning/optimization tools for PVT systems.

*Manufacturers*: Development of improved PVT system types and prefabricated components for PVT systems, such as PVT panels, heat pumps, storage, etc.

*Project planners, consultants, decision-makers, energy planners, property/real-state owners and construction/building contractors*: Education on PVT systems and different PVT demonstration systems in different locations followed for many years to document high reliability, high performance and long lifetime of PVT systems.

*Installers*: Installer education on PVT systems.

#### **1.8 The legal face of PVT solar collectors—European incentives**

The PowerUp MyHouse project aims to increase the knowledge and awareness about solar energy applications and in particular about PVT technologies. The project aims to investigate the best technological applications, manufacturing, installation, measures, calculations, legislation, incentives, supports, qualifications, experiments and vocational education related to PVT.

The project has been divided into *PVT Technologies Research*, which aims to present the latest researches done in and out of the European Union (EU) related to PVT technologies. *The legal face of PVT* aims to present legal arrangements on PVT done in well-developed countries such as legislation and incentives. A *Guidebook* in which the project will test a PVT system. Finally, the *learning module on PVT* is focused on vocational training for students in RES.

The PVT technology is still very recent in commercial terms, so the existing legislation is not suitable or does not explicitly contemplate all these types of solar systems. Furthermore, there are not many references about legislation applicable to PVT technology. Thus, given the scarce information on the legal framework for PVT systems, this document addresses this issue at the level of renewable energy system (RES) systems for the production of both electricity and heating, which are widely disseminated.

For PVT systems to have a fair chance compared to PV systems, special high subsidy levels for PVT systems are suggested for the following years. It is estimated that these subsidies might generate a sustainable market for PVT and PV systems.

The O1 output from the EU project PowerUp MyHouse on PVT Technology Research—Best practices report [22] suggests several subsidy scheme principles, such as:


The existing support and incentives, both for RESelectricity and RESheat, in the different countries, addressed in this document, are significantly different both in terms of amounts and in terms of the diversity of financial mechanisms. On the other hand, in some cases, there is the possibility that PVT systems are covered by the same supports as one or even both RESelectricity and RESheat systems.

In the countries of the EU, the next developments and opportunities in the renewable energy sector depend heavily on renewable energy development plans linked to the objective of reducing greenhouse gas emissions and the fulfilment of the commitments assumed under the Paris Agreement [23]. These renewable energy development plans are governed by two main regulations:

	- a. To cut greenhouse gas emissions by 40% in relation to 1990 levels;
	- b. 32% of final energy consumption to come from renewable sources;
	- c. An increase in the energy efficiency of 33%.

government included specific objectives in its respective document. It appears that in some of them they seem significantly more ambitious than those defined by the EU.

In this sense and according to the Solar Thermal World most recent publication [24], the main challenges facing the PVT industrial sector are:


Decisively, the Recovery and Resilience Facility aims at supporting reforms and investments undertaken by the Member States, which aids the economic and social impact mitigation of the Covid-19 pandemic. Moreover, these programs tend to strengthen the European economies and societies by means of a more sustainable, resilient and better prepared economic and social structure to combat the challenges and take the opportunities of the green and digital transition.

However, for the PVT technology to grow significantly outside its market niche, amongst other necessary actions, it is recommended to take the following measures [27]:

• Players must develop clever and fair support schemes for PVT collectors and systems, present them to governments around the world, and request their implementation. After all, the PVT sector does not receive nearly as much support as the PV or solar thermal industry.

*Photovoltaic-Thermal Solar Collectors – A Rising Solar Technology for an Urban… DOI: http://dx.doi.org/10.5772/intechopen.104543*

• Enlarging the knowledge of architects, planners and installers about PVT solutions. This should be helped by the fact that PVT is more efficient than just PV and is an attractive alternative to air and ground heat pumps.

Moreover, and following the previously stated incentives for solar systems, the European Union's Horizon 2020 research and innovation program leads the incentives for funding these types of solar technologies, such as the RES4BUILD, which develops RES-based solutions for decarbonising the energy use in buildings (e.g., new or renovated, tailored to their size, type and the climatic zones).

The project adopted a co-development approach, in which all the intervenients are involved in a dynamic process. Moreover, and in parallel, a full life cycle assessment (LCA) and life cycle economics (LCE) analysis are carried out, which aims at presenting the real impact of each proposed design. The diverse consortium and the dedicated exploitation tasks will connect the project with the market, paving the way for a wide application of the developed solutions.

Furthermore, the European Union's Horizon 2020 research and innovation program also provided financing for the development of PVT systems like the one presented by the RES4LIVE consortium, which adapts RES technologies, machinery and their demonstration at a large-scale on farm level. It requires supporting measures concerning spatial planning, infrastructure, different business models and market organisation, trends that are not all under control from a farmers' perspective.

The RES4LIVE project aims at fitting livestock farming with attractive costs, operational flexibility and low maintenance. The key technologies include integrated heat pumps, PVT solar collectors, PV panels, geothermal energy, electrification of on-farm machinery and biogas to be replaced by biomethane to fuel the retrofitted tractors.

Moreover, the PVT technology will be preferably installed on rooftops without occupying agricultural land. Focus on the collector mounting, piping and installation procedures to reach standardised solutions for livestock farming through (1) reducing the PVT system installations costs by more than 40%, and (2) by simplifying the installation process, to be handled by non-specialised technicians.

#### **2. Conclusions**

Due to the combination of both PV and ST technologies in the same gross area, PVT technologies employ the benefits of cooling the PV cells, thus increasing their overall efficiency, and thus using this excess heat to increase the HTF temperature of a solar thermal system.

Cabral [2] showed that PVT technologies have the potential to be a viable solution to compete with isolated systems such as PV and ST solar collectors if a significant reduction in manufacturing cost is achieved, coupled with an increased energy production performance. Therefore, its success is intensely linked to the capacity of the PVT industry/researchers to scale down its current system cost and complexity in a way that can shorten the cost/performance gap between both PV and ST technologies. PV and ST technologies have several decades of constant development; especially the PV industry with an exponential performance increment and constant decrease in material costs has been registered in the past decades.

Additionally, a bifacial PVT receiver comprising PV cells, which are high emitters, could potentially be equipped with a selective surface (i.e., between the PV cells) to increase the thermal energy yield through higher thermal efficiency.

The global production of silicon-crystalline-based PV modules typically entails a significant consumption of fossil fuels, which increases the dependence on non-RES sources. Conversely, the production of PVT collectors requires less energy. Other ways to increase useful thermal power at higher temperatures might rely on low-emissivity coatings either on top/bottom of solar glass covers or on top of the receiver core.

However, further studies are required to reduce reflectance and absorptance losses in the coatings. On the other hand, limited suitable highly transparent low-emissivity coatings are commercially available, which might limit a wider deployment of lowemissivity coatings in PVT collectors.

Furthermore, PVT technologies, in theory, allow the end-user to benefit from both feed-in tariffs and renewable heat incentives (RHI), due to the simultaneous production of electricity and heat. For DHW systems, in the UK, PVT technologies only benefit from the feed-in tariffs. However, for non-DHW systems, PVT technologies benefit from both incentives, which decreases the payback time.

In addition, the financial attractiveness concerning manufacturing and indirect costs can be improved by providing complete solar solutions with pre-configured packages of PVT collectors and auxiliary heating systems that facilitate the end user's decision. Moreover, the architectonic integration of PVT technologies into the building envelope offers a combined solution for both electricity and heat production while requiring less installation area.

An operational PVT system falls short in heat production when compared with a separate PV + ST system, which produces the majority of heat for DHW in the summer months. Whereas, in the winter months, when the required amount of DHW is not met, a backup system is required such as a heating component (i.e., boiler). This issue is also seen in ST systems, thus PVT technologies by also supplying electricity are on the verge of potentially being competitive, as PVTs are, at the core, ST collectors that can produce electricity from the same gross area. Therefore, it makes the future of PVT technology strictly reliant on the future of ST technologies, which rely on energy efficiency, durability and reliability aspects of a collector development.

The current global transformation of energy systems based on fossil fuels to RE systems lies predominantly on a high share of electrical power generation. The aim of reaching the required share of heating and cooling via power generation by the end of this century seems unrealistic with today's progress, which will require sustainable solutions such as ST and therefore PVT collectors.

The electrical storage trend is already ongoing through electrical batteries; nonetheless, heat tends to be easier, cheaper and environmentally friendlier to store than electricity, as it already gave real proof of its maturity and efficient reliability. In this way, PVT technologies have an opportunity to increase their market share, not neglecting permanent developments, both in terms of performance and in terms of cost reduction.

The high share of greenhouse gas emissions in the heat sector requires severe and methodical decarbonisation by a balanced technology mix, in which ST and PVT technologies must be considered, as it is crucial to achieve the already proposed goals in several environmental agreements.

#### **Acknowledgements**

The author would like to express his gratitude to the funding authorities that supported this work with funding, such as

*Photovoltaic-Thermal Solar Collectors – A Rising Solar Technology for an Urban… DOI: http://dx.doi.org/10.5772/intechopen.104543*


Furthermore, the author would like to appreciate the support and knowledge exchange from both Prof. Dr. Björn O. Karlsson and Dr. João Gomes.

#### **Acronyms and abbreviations**


#### **Appendices and nomenclature**


#### *Urban Transition – Perspectives on Urban Systems and Environments*


### **Author details**

Diogo O. Cabral Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, University of Gävle, Sweden

\*Address all correspondence to: diogo.cabral@hig.se

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Photovoltaic-Thermal Solar Collectors – A Rising Solar Technology for an Urban… DOI: http://dx.doi.org/10.5772/intechopen.104543*

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[2] Cabral D. Reflector Optimization for Low Concentration Photovoltaic-Thermal Solar Collectors, PhD thesis. Gävle: Gävle University Press; 2022. Available from: https:// www.diva-portal.org/smash/get/diva2: 1612644/FULLTEXT01.pdf

[3] Fraunhofer ISE. Photovoltaics Report. Updated in 30 October 2020. 2020

[4] Aste N, Leonforte F, Del Pero C. Design, modeling and performance monitoring of a photovoltaic–thermal (PVT) water collector. Solar Energy. 2015;**112**:85-99

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[6] Lämmle M. Thermal Management of PVT Collectors: Development and Modelling of Highly Efficient Glazed, Flat Plate PVT Collectors with Low-Emissivity Coatings and Overheating Protection, PhD thesis. Freiburg: Fraunhofer ISE; 2018

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[13] Zondag HA, de Vries DW, de Van Helden WGJ, Van Zolingen RJC, Van Steenhoven AA The thermal and electrical yield of a PV-thermal collector. Solar Energy 72, 2002, 113-128

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[17] ISO9806:2017 ISO 9806:2017. Solar Energy-Solar Thermal Collectors-Test Methods. Geneva, Switzerland: ISO; 2017

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[22] Perers B, Furbo S, Dragsted J, Hayati A, Cabral D, Gomes J, Kaziukonytė J, et al. O1 PVT Technology Research - Best Practices Report. EU Project Power Up My House, 2021

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#### **Chapter 7**

## Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements

*Alireza Bahrami, Arman Ameen and Henry Nkweto*

#### **Abstract**

The energy use of buildings is gradually increasing, which is due to economic growth and an increase in population. Several studies have indicated that the implementation of energy-saving measures (ESMs) such as thermal insulation results in more energy saving; however, most ESMs are not economically viable. This chapter outlines ESMs using the IDA ICE computer software. The evaluation of the energy performance of two multifamily buildings is conducted, and possible ESMs are suggested such as thermal insulation, changing windows, installing a new air handling unit, installing a heat exchanger in showers, improving thermal bridges, replacing lighting bulbs, increasing external insulation plus temperature reduction, and changing schedules for air discharge control. The economic feasibility of these suggestions is assessed using the life cycle cost analysis to determine their economic viability. This involves the determination of the life cycle cost and life cycle cost saving to decide the best option. The most important factor in determining life cycle cost saving is the modified uniform present value. The addition of the attic insulation, installing a heat exchanger in showers, replacing lighting bulbs, and changing schedules meet the economic requirement within a feasible time frame.

**Keywords:** energy audit, energy-saving measure, economic analysis, life cycle cost, life cycle cost saving, energy efficiency, IDA ICE, multifamily buildings

#### **1. Introduction**

Energy plays an important role in economic growth and other daily humans' activities. Global energy use is mainly supplied from fossil fuels such as oil, gas, and coal. This accounts for 80% of the total energy use worldwide. The share of fossil fuels includes 33% of crude oil, 27% of coal, 22% of natural gas, and 18% of other sources [1]. Fossil fuels are not useful energy resources owing to their limited availability and impact on climate. In recent years, energy prices are increasing. This is due to the gradual increase in economic and population growth. To reduce this impact, the United Nations has considered essential measures through Paris Agreement to combat climate change. The central aim of the agreement is to strengthen the global response to climate change by keeping the temperature rise below 2°C [2].

Global energy use is mainly divided into three sectors which include industry, transportation, and residential and service buildings. Both the industry and building sectors show a gradual increase when compared with transport [3]. The increased environmental awareness and energy analysis of buildings are the tools that would drive the design of buildings with low environmental impact and energy use.

Buildings are made of enclosure that separates the internal environment from the external. Energy is used for lighting, cooking, running appliances, thermal comfort, and many other applications in buildings. Energy use in the building sector is rapidly growing. This may cause a serious environmental problem [4] in Sweden and a challenge for the European Union's (EU) directions. The energy used by buildings is approximately 40% of the total energy in the EU [5]. To reduce this, the EU proposed a directive on the energy performance of buildings, and this was implemented in 2006. The main purpose of the directive is to improve the total energy efficiency of buildings. This includes new and existing buildings.

In the EU, energy use of buildings is becoming the fastest-growing sector. Energy is needed for various purposes which include thermal comfort, lighting, cooking, etc. The need for energy saving is of great significance especially considering the fluctuation in energy prices, and the population and economic growth [6]. In this study, Doukas et al. evaluated the decision-making process for selecting energy-saving measures (ESMs). The systematic approach was integrated based on key areas of energy management systems of buildings such as load, demand, and user requirements.

An energy investigation was carried out on a multifamily building in Sweden [7]. There a simulation of the building was conducted by using IDA ICE. The results included various ESMs and analysis of the individual measure elaborated that the building had the potential to reduce the energy by 50%. This would further reduce CO2 emissions by more than 43.3%.

Air leakages through building elements can result in a change in temperature [8]. The research performed in [8] presented a critical review of the use of the infrared thermography (IRT) survey in the building energy audit. IRT identifies leakages and thermal bridges. It was indicated that after identifying the leakages when used together with the blower door method in a building and then applying retrofitting measures, they would result in substantial energy reduction for the building.

An energy audit conducted in [9] using IDA ICE demonstrated that lowering indoor temperature could provide advantages when reducing energy use. However, lowering the indoor temperature should be combined with the insulation of the external wall. This retrofitting as a package could achieve a 53.3% reduction in the total energy delivered.

Studies [10, 11] reported that one of the biggest wastes of energy was caused by inefficient lighting. Moreover, lighting accounts for a great part of the total energy use in buildings. Using energy-efficient lights with demand and proper daylighting controls could help reduce electrical demand. This would contribute to visual comfort and green building development. Furthermore, it was illustrated that LED lighting systems reduced total power use by up to 21.9% [10]. However, in most apartments, human behavior and switching on/off depending on the need played an important role in selecting a more efficient light.

The energy audit carried out on a Swedish multifamily building using IDA ICE presented a change in the overall heat energy demand for ventilation when demand control was used [9]. This resulted in an approximately 50% reduction in the annual heating demand. Several studies [12, 13] suggested that when a heat recovery system *Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements DOI: http://dx.doi.org/10.5772/intechopen.109940*

was utilized in buildings, it was done by the air handling unit (AHU) which recovered heat from the exhaust air.

The selection of ESMs depends on the capital investment and benefit achieved after the implementation of ESMs. Various economic analysis methods can be utilized to evaluate the economic viability of each ESM [8]. However, there are various ways to analyze the economic feasibility of ESMs generated from an energy audit. Several studies [7, 8] employed life cycle cost to assess the profitability of ESMs. Life cycle cost consists of investment, energy, and maintenance costs. According to [7], when assessing which ESM is the most profitable, the outcomes are achieved when the life cycle cost is lower than the life cycle saving.

It is also important to link ESMs to environmental urban transition and urban building energy conservation [14] since reducing energy usage in buildings in urban area contributes greatly to a sustainable and environmental urban transition.

This chapter focuses on the energy audit of two multifamily buildings owned by the company Älvkarlebyhus AB located in Skutskär, Sweden. Also, some ESMs are proposed for these buildings such as thermal insulation, changing windows, installing a new AHU, installing a heat exchanger in showers, improving thermal bridges, replacing lighting bulbs, increasing external insulation plus temperature reduction, and changing schedules for air discharge control. These ESMs have also been promoted in research [15–17]. The life cycle costs of these proposed ESMs are analyzed and discussed.

#### **2. Materials and methods**

The following approach was adopted in this research. Firstly, the research involved ventilation measurements and data collection of the actual building. This includes the design plans of the buildings, site, materials, ventilation systems, energy use for hot water, and energy bills. This was further used as the input in IDA ICE for the simulation and validation of the base models. Lastly, an IR-thermal camera was utilized to detect leakages.

#### **2.1 Field study objects**

The study was conducted on two buildings owned by the housing company named Älvkarlebyhus AB located in Skutskär, Sweden. The company is publicly owned. The company is taking ESMs to decrease the energy use in these two multifamily buildings. The buildings are Centralgatan 14 and Tebogatan 5 which were constructed in 1968. Centralgatan 14 is both a residential and service building. It has five business shops on the first floor and sixteen apartments on the others. While Tebogatan 5 is a residential building with eighteen apartments. Heat is supplied by the district heating (DH) company Bionär AB, while electricity is supplied by Vattenfall AB. Centralgatan 14, Tebogatan 5, and Tebogatan 6 share one central heating system. The total heating energy demand for all the buildings is 710.082 MWh/year obtained from the invoices. According to the financial accountant of the company, the energy demand for each building is obtained from the ratio 2:2:1, respectively, as recommended by the DH company. The buildings of this study are displayed in **Figures 1** and **2**.

**Figure 1.** *Studied building,Tebogatan 5.*

**Figure 2.** *Studied building, Centralgatan 14.*

#### **2.2 Field measurements**

The measurements were done in Centralgatan 14 and Tebogatan 5. Ventilation flow rates were measured using electronic instruments. Two sets of instruments were utilized which were the hot wire anemometer (VelociCalc Plus 8386, TSI incorporated Ltd.) and hood (Testo 420, Swena flow air hood). The airflow hood is an electronic air instrument that was used for air volume measurement passing through the mechanically ventilated duct (uncertainty 6% l/s) [7]. The hot wire anemometer measured the speed per second (uncertainty 0.1 m/s). **Figure 3** illustrates the measuring instruments employed for the field measurements.

#### **2.3 Inspection of thermal bridges and leakages**

To identify the thermal bridges and leakages in the buildings, an IR camera (Thermal CAM S60, FLIR Systems) was used. This is because the air infiltration in *Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements DOI: http://dx.doi.org/10.5772/intechopen.109940*

**Figure 3.** *Ventilation measuring devices; thermo-anemometer (TSI) on the left side and airflow hood on the right side.*

buildings cannot be seen by visual inspection in most cases. Therefore, special equipment is needed to detect them. The IR-thermal camera is a fast and reliable tool to identify leakages in buildings [15]. The inspection of thermal bridges and leakages was conducted which demonstrated that the leakages were distributed on the buildings' envelopes exposed to the external environment. **Figures 4** and **5** indicate examples of IR thermography pictures collected during the inspection.

#### **Figure 4.**

*IR-thermography picture showing thermal bridges around the window in Centralgatan 14.*

**Figure 5.**

*IR-thermography picture showing thermal bridges around the entrance of Centralgatan 14.*

#### **2.4 IDA ICE simulation models**

The models for Centralgatan 14 and Tebogatan 5 were built in the IDA ICE simulation software. IDA ICE is an important computer tool for the simulation and optimization of buildings' energy use [9]. The process of modeling involved importing the architectural drawings of each level of the buildings. Then, the zones representing each apartment within the buildings were created. The required information about the buildings was taken from the drawings. This included the dimension of the buildings, the size of the windows, the height of the buildings, and other required inputs. Söderhamn's (Sweden) weather file was utilized in the software. Then, the boundary conditions which were needed to be adopted in the software were building materials, lights and equipment, air leakage areas, ventilations, occupants of the buildings, weather data, indoor and outdoor temperatures, and the ventilation system and the temperature in corridors. **Figures 6** and **7** display the base models created in IDA ICE for Tebogatan 5 and Centralgatan 14, respectively.

**Figure 6.** *IDA ICE model created for Tebogatan 5.*

**Figure 7.** *IDA ICE model created for Centralgatan 14.*

*Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements DOI: http://dx.doi.org/10.5772/intechopen.109940*

#### **2.5 Parameters of buildings materials**

**Table 1** lists the materials that were used as the input to the models of Tebogatan 5 and Centralgatan 14. Apart from the wall surface, windows consisted of wood, covering the areas between windows and the surrounding surface.

#### **2.6 Other input parameters**

Other input parameters considered for the models are explained here.

#### *2.6.1 Ventilation system*

The base model was developed using a mechanical exhaust ventilation system for Tebogatan 5. Centralgatan 14 has both a supply and exhaust unit with a heat exchanger for the shops and a mechanical exhaust ventilation unit for the apartments. In the exhaust type of ventilation system, no supply is required. In this system, the air was entering through adjustable slots located on top of the windows.

#### *2.6.2 Temperature and air infiltration of buildings*

The room temperature of the models was set at 20°C. The air infiltration rate was taken as 0.36 l/(s.m2 ) at 50 Pa.

#### *2.6.3 Ground properties*

The ground specification was modeled using ISO standard 13,370. The basement floor was taken as 200 mm concrete, and the ground layer outside the basement was assumed by default values having a ground layer of 0.1 m.


**Table 1.** *Materials of buildings.*

#### *2.6.4 Internal gains and masses*

The occupancy, lighting system, and equipment are among the sources of internal gain in a building. The activity levels in the created zones were assumed as 1MET, and the number of people in each apartment was determined using SVEBY standard: 1rk = 1.42, 2rk = 1.63, 3rk = 2.18, 4rk = 2.79, and 5rk = 3.51. Therefore, it involved counting the number of bedrooms creating a zone, and depicting the number from the standard to set it as the input to the model. The occupancy was assumed to use the apartments 14 hours/ day. The lighting and equipment used electricity, and 70% of that energy was converted to heat. In addition, occupants contributed to the internal heat gain.

The portion of stairs and the internal mass was assumed as an area of 2.72 m<sup>2</sup> per stair height with the heat transfer coefficient of 1.7 W/mK of concrete.

#### *2.6.5 Heating and cooling systems*

Each apartment was modeled using the existing ideal heaters in the modeling software, and no cooling system was present. The heating system was provided by DH and supplied to the buildings.

#### *2.6.6 Assumptions*


#### **2.7 Economic analysis**

After proposing ESMs, an economic evaluation was carried out on the measures taken. This was performed to ensure that savings were at least greater than the

*Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements DOI: http://dx.doi.org/10.5772/intechopen.109940*

investments. Life cycle costs were determined using the equation of the net present value (NPV), Eq. (1). NPV is generally used to calculate the cost when it is evenly distributed during *n* years. It takes into account the parameter *r* as the interest rate and *p* as the estimated rate of the increase in energy prices. Therefore, the net interest rate between the real interest rate and real energy price, *f*, is expressed as in Eq. (2) which is utilized in NPV, Eq. (1) [7]:

$$NPV = \frac{(\mathbf{1} + f)^n - \mathbf{1}}{f(\mathbf{1} + f)^n} \tag{1}$$

$$f = \frac{r - p}{1 + p} \tag{2}$$

#### **3. Results and discussion**

The results obtained from the analysis are represented and discussed in this section.

#### **3.1 Validity of base models**

Based on the simulation results, the total heating loads for Tebogatan 5 and Centralgatan 14 were 263.2 MWh/year and 301.2 MWh/year, respectively. The results from the models are summarized in **Table 2**. They were compared with the real value of the total consumption of 284 MWh/year for heating and domestic hot water of both buildings. The reference value for the heating demand was obtained from the calculations (**Table 2**). This consisted of domestic hot water and the energy used for zone heating. The table also gives the total electricity demand in a year for both buildings. As can be seen from the table, the percentages of the errors between the reference values of the real buildings and the simulation results were acceptable which validated the modeling results with good accuracy.


#### **Table 2.**

*Comparison of base models with reference values.*

#### **3.2 Energy balance**

Energy balance is one of the most important parts of the verified base models. It allows identifying the influence of each part of the buildings contributing to high losses to the buildings. It is also the key factor in implementing retrofitting measures. **Figures 8** and **9** display the energy balances of the base models. They allow for the identification of the specific areas required to reduce heat losses. They illustrate that

**Figure 8.** *Energy balance for Tebogatan 5.*

**Figure 9.** *Energy balance for Centralgatan 14.*

higher losses were caused by the buildings' envelopes. In addition, the heat was lost through the air created by the ventilation system. It was needed to improve the insulation of the buildings' envelopes to reduce the high energy losses.

#### **3.3 Transmission loses**

Heat losses are transferred from buildings through different parts. **Figures 10** and **11** illustrate huge transmission losses through the buildings' envelopes, especially through the walls, windows, and roof. Thermal bridges such as balconies slabs also contributed to the additional heat losses. From the energy balance of both buildings, it was observed that the largest share of heat losses was due to the buildings' envelopes and thermal bridges. Apart from the transmission losses taking up the largest heat losses, some additional heat losses were contributed by the air infiltration of the buildings. Identifying the air infiltration of the buildings required the combination of

*Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements DOI: http://dx.doi.org/10.5772/intechopen.109940*

#### **Figure 11.**

the IR-thermal camera and blower door method. In the study, as observed on the IRthermal camera, the result demonstrated that thermal bridges were concentrated on the joints formed by the external walls and doors. Implementing retrofits is an effective solution that can promote energy saving. Therefore, various ESMs were needed to reduce heat transmission losses through the buildings and to optimize energy use.

#### **3.4 Energy-saving measures (ESMs)**

The energy performance was evaluated to improve the overall energy efficiency of the buildings. Several ESMs were proposed to optimize the energy use of both buildings. They included thermal insulation (increasing both external wall and attic insulations), changing windows, installing a new AHU, installing a heat exchanger in showers, improving thermal bridges, replacing lighting bulbs, increasing external insulation plus temperature reduction, and changing schedules for air discharge control.

*Transmission losses for Centralgatan 14.*

#### *3.4.1 Scenarios A1 and A2: thermal insulation*

The addition of external insulation to buildings results in high energy savings. It reduces the flow of heat out of buildings and promotes energy saving of buildings [6]. Thermal insulation was added to the external walls and roof to provide an effective ESM that reduced the overall heat loss of the buildings [19]. On both buildings' models, 200 mm mineral wool insulation (0.036 W/mK) was added to the external surface. The results indicated a decrease in the energy use for heating in both buildings. The net saving of energy by adding 200 mm of external insulation was 10% for both Centralgatan 14 and Tebogatan 5. To reduce the heat loss through the roof, attic insulation was utilized with a thermal conductivity of 0.036 W/m.K and a thickness of 200 mm. The net energy saving for the heating energy demand was achieved at 6% for Centralgatan 14 and 7% for Tebogatan 5. These energy savings are listed in **Table 3**.


**Table 3.**

*Energy saving when thermal insulation was implemented.*

#### *3.4.2 Scenario B: changing windows*

The windows were changed by replacing them with a lower U-value (U = 0.85 W/ m2 K). The energy saving for the heating energy demand after replacing the windows was 10% for Centralgatan 14 and 7% for Tebogatan 5, as presented in **Table 4**. The net losses through the windows were reduced.


**Table 4.**

*Energy saving when windows with U = 0.85 W/m<sup>2</sup> K were erected.*

#### *3.4.3 Scenario C: installing a new air handling unit*

Various control systems are employed in the AHU. The base model utilized constant air volume (CAV) on the mechanical exhaust system. This system requires high flow rates and higher energy for heating. This system was replaced with a standard AHU having variable air volume (VAV) with temperature control and a heat recovery exchanger with an efficiency of 85%. This is more appropriate for achieving good thermal comfort as well as reducing energy usage in buildings. The net energy savings of heating energy demand was 30% for Centralgatan 14 and 34% for Tebogatan 5 when compared with the base models. **Table 5** summarizes the percentages of energy saving by installing a new AHU.

*Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements DOI: http://dx.doi.org/10.5772/intechopen.109940*


**Table 5.**

*Energy saving by installing a new AHU.*

*3.4.4 Scenario D: installing a heat exchanger in showers*

The installation of a heat exchanger in the showers resulted in reduced hot water use by 20%. When this was implemented, the heating energy demand was reduced and led to a net saving of 5% for Centralgatan 14 and 6% for Tebogatan 5 (**Table 6**).


**Table 6.**

*Energy saving by installing a heat exchanger in showers.*

#### *3.4.5 Scenario E: improving thermal bridges*

The presence of thermal bridges in buildings' envelopes affects the energy use and thermal comfort of occupants. To reduce the typical leakages displayed in **Figures 4** and **5**, it was required to change external windows and doors to the ones with low thermal bridges. Thermal bridges are parts of the buildings' envelopes that have a major effect on thermal performance [20]. When the thermal bridges were improved, the energy demand for the zone heating was reduced with a net saving of 5% for Centralgatan 14 and 3% for Tebogatan 5 compared with the base models, as indicated in **Table 7**. The comfort of the buildings remained the same as the base models.


**Table 7.**

*Energy saving by improving thermal bridges.*

#### *3.4.6 Scenario F: replacing lighting bulbs*

The lighting bulbs utilized in the buildings were fluorescent and candescent based on the site visits. The average power rating for the lamps is 60 W in the apartments. To improve the efficiency of the buildings' lighting systems, the lighting bulbs should be upgraded to energy-saving bulbs. When this was replaced by 20 W LED lighting with the same luminous flux, it would lead to an electrical energy saving of 21.3% for Tebogatan 5 and 24.2% for Centralgatan 14 (**Table 8**). Using LED bulbs resulted in a


#### **Table 8.**

*Energy saving by replacing lighting bulbs.*

small increase in the heat demand for both buildings. However, the amount of energy savings obtained from implementing this ESM was higher when compared with the heat demand.

#### *3.4.7 Scenario G: increasing external insulation plus temperature reduction*

The indoor temperature was lowered by 2–18°C. Maintaining the energy balance for the buildings needed increasing the external insulation to 200 mm. The total energy saving was 20% for Centralgatan 14 and 21% for Tebogatan 5 compared with the base models. This energy saving is reported in **Table 9**. The change was the result of reduced heating value for the zones, while the domestic hot water remained the same. Though, this ESM had a high impact on thermal comfort.


**Table 9.**

*Net energy saving by external insulation plus temperature reduction.*

#### *3.4.8 Scenario H: changing schedules for air discharge control*

The schedule for the air discharge control is different from VAV because it works on the demand control principle. Air is extracted depending on the demand inside the buildings. The schedule was changed during the period when the demand was low. The supply of heating energy demand was reduced because of the reduced heat generated. Therefore, running the ventilation system when it is not required, results in high energy use. When the schedules were integrated into the air discharge control, it led to a heat energy saving of 14% for Centralgatan 14 and 12% for Tebogatan 5. These percentages of energy saving are provided in **Table 10**.


#### **Table 10.**

*Energy saving by changing schedules for air discharge control.*

#### **3.5 Summary of ESMs and possible outcomes**

**Table 11** summarizes the energy savings when the mentioned ESMs were implemented.

*Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements DOI: http://dx.doi.org/10.5772/intechopen.109940*


**Table 11.**

*ESMs and possible energy savings.*

#### **3.6 Thermal comfort for base models**

IDA ICE integrates many standards which include ISO 7730 for computing the thermal comfort of buildings. From the analysis of the base models, the predicted percentage of dissatisfied (PPD) index was 9% for the base model of Centralgatan 14 and 14% for Tebogatan 5. This is within the acceptable standard of EN 15251. When the operative temperature was above 27°C, the percentage of hours was 3% for Centralgatan 14 and 1% for Tebogatan 5. **Table 12** illustrates PPDs for the base models.


**Table 12.** *PPD for base models.*

#### *3.6.1 Effect of ESMs on thermal comfort*

Using the proposed ESMs in the base models to improve the energy efficiency of the buildings led to changes in thermal comfort. The PPD of the base models was increased depending on the implemented ESMs. When it was compared with the EN ISO 7730 which states that the acceptable thermal dissatisfaction in buildings should be smaller than 15%, various measures were therefore within the limit.

ESMs introduced in the models resulted in substantial energy savings for the buildings. The use of these measures had less impact on thermal comfort, apart from the combined effect of adding the external insulation plus lowering the indoor temperature. The combined effect of adding the external insulation plus lowering indoor temperature affected the thermal comfort considerably. The percentage

**Figure 12.** *Thermal comforts after implementation of each ESM and their comparison with the standard requirement.*

of the total occupant hours with thermal dissatisfaction increased from 8% to 15% in Centralgatan 14, and from 8% to 28% in Tebogatan 5. This occurred when the external insulation was increased by 200 mm plus lowering the temperature by 2°C.

The percentage of occupants with thermal dissatisfaction was reduced to 13% as compared with 14% of the base model when the external insulation was added for Tebogatan 5, while it remained the same (9%) for Centralgatan 14. However, when the operative temperature was above 27°C, the percentage of hours was increased from 3% to 8% for Centralgatan 14, whilst it remained the same (1%) for Tebogatan 5.

Reducing the U-value to 0.85 W/m<sup>2</sup> K for the windows led to PPD remaining the same as the base models, 14% for Tebogatan 5, but it was increased from 9% to 10% for Centralgatan 14. When the operative temperature was above 27°C, the percentage of hours was increased from 3% to 5% for Centralgatan 14.

When AHU with VAV plus temperature control was used for both buildings, the percentage of the total occupant hours with thermal dissatisfaction was increased by 1% for both buildings. However, the percentage of hours, when the operative temperature was above 27°C in the worst zone, was increased from 1% to 2% for Tebogatan 5. **Figure 12** depicts the summary of the thermal results after implementing each ESM in the base models.

#### **3.7 Economic feasibility of ESMs**

The implementation of ESMs largely depends on capital, and it should be analyzed in such a way that the investment could be viable. Therefore, it is important to optimize energy use by improving areas with huge heat loss. In the base models, the heat was mainly lost through walls, windows, and thermal bridges of roofs and floors. After implementing ESMs, it was important to carry out the economic feasibility of ESMs. Life cycle cost determines the most cost-effective approach from a series of alternative ESMs.

*Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements DOI: http://dx.doi.org/10.5772/intechopen.109940*

The life cycle analysis was done based on Eq. (3). This equation is the modified uniform present value (NPV. Factor) which is obtained by modifying NPV [21]:

$$\text{Modified uniform present value } (\text{NPV.Factor}) = \frac{(\mathbf{1} + r - p)^n - \mathbf{1}}{(r - p)(\mathbf{1} + r - p)^n} \tag{3}$$

where *p* is the estimated rate of increase in either electricity or DH, and *r* is the interest rate.

**Tables 13** and **14** provide life cycle costs achieved for both buildings and indicate which ESMs are economically viable. Despite high energy savings by implementing certain ESMs, the results demonstrated that some ESMs were not economically viable. This was because of higher life cycle costs compared with their life cycle savings. ESMs that were not viable for both buildings include adding 200 mm external insulation, replacing windows with less U-value, installing a new AHU, improving thermal bridges, and increasing external insulation plus temperature reduction. However, according to the tables, adding 200 mm attic insulation, installing a new heat


#### **Table 13.**

*Results of life cycle cost analysis for Centralgatan 14.*


#### **Table 14.**

*Results of life cycle cost analysis for Tebogatan 5.*

exchanger in showers, replacing lighting bulbs, and changing schedules for both buildings represented economical ESMs. This was thanks to lower life cycle costs than life cycle savings.

#### **4. Conclusions**

The buildings' envelopes and thermal bridges greatly affected the energy performance of the buildings. Most of the heat losses in the buildings occurred through them. To analyze the buildings' performance, various ESMs were studied to demonstrate how the energy efficiency of the buildings would be improved. The addition of thermal insulation, changing windows, installing an AHU, installing a heat exchanger in the showers, improving thermal bridges, and changing air discharge schedules could improve the energy performance of the buildings. The suggested ESMs affected the buildings' energy performance positively. All the implemented ESMs contributed to energy saving for the buildings, but their economic feasibility depended on the economic aspect of their life cycle. This was based on the investment cost and life cycle savings of ESMs. The amount of energy reduction from ESMs varied; the combination of external insulation plus temperature reduction had the highest impact on the energy reduction of the buildings. However, from the individual ESM, the highest energy reduction was recorded from the analysis of installing a new AHU, and this reflected the importance of ventilation with a heat recovery system in the buildings. The economically viable ESMs included the addition of attic insulation, installing a heat exchanger in the showers, replacing lighting bulbs, and changing the schedules for AHU control.

This study links ESMs in existing buildings to urban sustainable development and energy efficiency in buildings. Since many buildings are old, they do not meet the requirements of being energy efficient according to the latest building codes or contributing to urban sustainability, however, this can be bridged with the implementation of ESMs.

#### **Author details**

Alireza Bahrami\*, Arman Ameen and Henry Nkweto Department of Building Engineering, Energy Systems and Sustainability Science, Faculty of Engineering and Sustainable Development, University of Gävle, Gävle, Sweden

\*Address all correspondence to: alireza.bahrami@hig.se

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements DOI: http://dx.doi.org/10.5772/intechopen.109940*

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### Section 3
