**2.4. Separate outdoor and indoor environments by means of a thermal barrier if no physical limit exists between them**

Through the openings of building walls and partitions there are inlets and outlets of air, whose direction and value depend, for each point of the envelope, on the pressure differences that exists at each of its sides. In the absence of mechanical systems, the pressure difference is due to the temperature difference between indoor and outdoor air (thermal draft) or to the wind pressure.

The airflow through an opening due to thermal buoyancy has been studied by different authors (Allard & Utsumi, 1992). The air flow rate *Q* in m3/s which enters through an opening h meters high and with a surface of *S* m2 when there is a temperature difference *Ti* - *Te* between the outside air and the inside air can be obtained from the following expression:

$$S = \frac{Q}{\int \varphi \, \rho\_0 \, T\_0 \left(\frac{1}{T\_\epsilon} - \frac{1}{T\_i}\right) z^{\int\_{\epsilon}^{\epsilon}}} \tag{5}$$

where *z* = *h*/2 (considering that the neutral pressure level passes through the centerline of the side opening), and *n* is the flow exponent that indicates the degree of turbulence. An *n* value of 0.5 represents fully turbulent flow and 1.0 represents fully laminar flow.

From the volume flow rate obtained, the sensible and latent losses (or gains) are calculated using the well known expressions:

$$\mathbf{q}\mathbf{s} = \mathbf{Q} \cdot (\mathbf{T}\_{\mathbf{e}} \text{ - Ti}) \cdot \mathbf{Q} \mathbf{a} \text{ - c} \tag{6}$$

$$\mathbf{q}\_{\text{v}\to\text{V}} = \mathbf{Q} \cdot (\mathbf{w}\_{\text{e}} \text{ - } \mathbf{w}\text{i}) \cdot \mathbf{q}\_{\text{a}} \text{ - L} \tag{7}$$

Heating (or cooling) loads laws distribute at both sides of the neutral pressure level according to the Figure 1:

**Figure 1.** Heating loads due to thermal buoyancy through openings

110 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

needed by the boiler x Coefficient for the primary energy used.

two energy conversion factors, for carbon emissions and primary energy.

**Table 2.** Carbon emission and primary energy conversion factors used by CALENER

Therefore, a significant reduction of carbon emissions can be carried out in two ways. The first is to choose a primary source of energy with the lowest carbon footprint, eg. natural

**2.4. Separate outdoor and indoor environments by means of a thermal barrier if** 

Through the openings of building walls and partitions there are inlets and outlets of air, whose direction and value depend, for each point of the envelope, on the pressure differences that exists at each of its sides. In the absence of mechanical systems, the pressure difference is due to the temperature difference between indoor and outdoor air (thermal

The airflow through an opening due to thermal buoyancy has been studied by different authors (Allard & Utsumi, 1992). The air flow rate *Q* in m3/s which enters through an opening h meters high and with a surface of *S* m2 when there is a temperature difference *Ti* - *Te* between the outside air and the inside air can be obtained from the following expression:

0 0

*g T z T T*

 − 

<sup>=</sup>

*<sup>Q</sup> <sup>S</sup>*

ρ

1

(5)

1 1 *<sup>n</sup> e i*

**(kWh)** 

Electricity 1 2,603 0.649 Natural Gas 1 1,011 0.204 Coal 1 1 0.347 Liquefied petroleum gas 1 1,081 0.244 Diesel oil 1 1,081 0.287 Fuel oil 1 1,081 0.280 Biofuel 1 1 0 Renewable energy ¿1? ¿1? 0

**Type of energ**y **Final energy** 

gas. The second is to use a source of renewable energy.

**no physical limit exists between them** 

draft) or to the wind pressure.

conversion factor.

• Annual energy consumption of the primary fuel used in the boiler = Annual energy

• Amount of carbon emitted = Annual energy consumption of primary fuel x Carbon

The following Table 2 shows the conversion coefficients used by the official program for energy rating process in Spain, CALENER (IDAE, 2009). Each different energy source has

> **Primary energy (kWh)**

**Emissions (kg. CO2)** 

> According to the technical services prevailing code (R.D. 1027/2007, 2007), the design of a partially open building, with 8 side openings of 4x4 m2 each, which keeps the indoor environment between 19°C and 28°C throughout the year, would require a heating power close to 2.5 MW if the outdoor temperature were 5°C. In summer, sensible cooling power reaches 1.5 MW for an outside temperature of 35°C.

> At this point it should be emphasized the importance of the latent loads in warm and humid climates. In these cases, the use of direct expansion cooling machines leads to high electrical consumption for hardly maintaining the desired internal temperature conditions. This fact alone would justify the use of climate separators in these climates, for all type of buildings, provided that a high frequency of doors opening and closing is expected.

> It should also be noted that the infiltration air flow rate, and consequently the thermal load, is usually increased due to wind pressures acting on opposite faces of the building. In this case, even more so air curtains reveal as the essential strategy to reduce the thermal load of this type of buildings, for they create a barrier between the two environments, the inside working area of the workshop and the outside air. The air curtains also are highly energy efficient terminal units, and their coils permit a choice of the type of energy they run on, hot water included.

### **2.5. Reduce the causes of discomfort**

The degree of user acceptance of the environmental conditions of the premises is a function of their air quality, which includes higrothermal comfort and adequate pollution levels.

There is enough information about the influence of higrothermal conditions on the predicted percentage of dissatisfied (PPD) in a room (Fanger, 1993a). The expressions that relate the PPD with different causes of higrothermal discomfort are the following:

#### a. PPD due to vertical temperature gradient

The *PPD* as a function of temperature gradient is given by the equation (ASHRAE, 2009a):

$$PPD = \frac{100}{1 + e^{\left(5.76 - 0.856 \, TG\right)}} \tag{8}$$

High Efficiency Mix Energy System Design with Low Carbon Footprint for Wide-Open Workshops 113

**Element** Cold wall Cold roof Warm wall Warm roof **Maximum temperature differences** 11 16 35 6

With respect to air quality, the effect on the PDP of the contaminants perceived by the sense of smell has also been extensively discussed (Fanger, 1993b), but it does not when it comes to pollutants that are not detected by humans, for it is unknown its influence on the PPD

Fanger established how the percentage of people dissatisfied is influenced by the presence of pollutants, provided that they can be perceivable. For low concentrations (usual when working indoors) the relationship is linear. For low concentrations (usual working indoors)

Of pollutants detected by humans, one of the most common is carbon dioxide (CO2). The relationship between the *PPD* and the concentration of CO2 in the air can be determined

• For *PPD* = 100%, CO2 concentration is 10.000 ppm (350 ppm supplied by the outside

• For *PPD* = 15%, concentration is 850 ppm (500 ppm produced in the inside the premises,

Under the hypothesis that contaminants not perceived could be studied under the aforementioned theoretical frame, so that their effect on humans could be brought together and their contribution to the PPD could be analyzed, Gomez (2009) has developed a method for quickly and easily determine the PPD for any room with any type of pollution (detected or not by humans). It is based upon the concentrations of pollutants prescribed by international health agencies, for which there is adopted the type of curve derived by Fanger

ACGIH (1999) and the existing standards on health and hygiene at work suggest the maximum allowable concentration of pollutants for different periods of time TLV (Threshold Limit Values). According to them, being exposed to a specific concentration of pollutants for 8 hours a day leads to a *PPD* of 100%. This concentration is known as the

395

*PPD e*

where *C* is the carbon dioxide CO2 concentration in ppm.

• For *PPD* = 30%, *PPD*/CO2 relationship is almost linear.

From the previous expression it can be deduced:

and 350 ppm supplied by the outside air).

f. PPD due to any other pollutant

for CO2 as an average value.

TLWA value.

( ) 0.25 15.15· 350

<sup>−</sup> − − <sup>=</sup> (12)

*C*

**Table 3.** Values of asymmetric radiation for a PPD ≤10%

and how affects the workers productivity. e. PPD due to human detected pollutants

through the expression (Fanger, 1993c):

the relationship is linear.

air).

where *TG* is the vertical thermal gradient between head and feet.

b. Synergistic effect on PPD of air velocity, temperature and turbulence

The changes in the *PPD* by the synergistic effects of air velocity, temperature and turbulence, are defined by the expression (BS EN ISO 7730:2005, 2006a, 2006b):

$$\text{APPD} = (\text{34} - T\_2) \cdot (\upsilon - 0.05)^{0.62} \cdot (0.37 \cdot \upsilon \cdot T\_u + 3.14) \tag{9}$$

where: *Ta* is the air temperature (ºC), *v* is the air velocity (m/s) and *Tu* is the turbulence intensity (%), considering that the air turbulence intensity in a point is defined by the equation:

$$T\_u = \frac{\sigma\_v}{v} \tag{10}$$

where *σv* is the standard deviation and *v* is the mean air velocity of a random sample of velocities.

#### c. PPD due to inadequate floor temperature

The percentage of dissatisfied by warm or cold floor can be deduced through the following expression (BS EN ISO 7730:2005, 2006c), that relates the *PPD* to the floor temperature *tf*.

$$PPD = 100 - 94 \, e^{\left(-1.387 + 0.118 \cdot t\_f - 0.0025 \cdot t\_f^{-2}\right)} \tag{11}$$

#### d. PPD due to asymmetric thermal radiation

Asymmetric radiation from warm or cold surfaces, created by high lighting levels, due to large glazed surfaces or direct sunlight can reduce thermal acceptability of the spaces (BS EN ISO 7730:2005, 2006d). Radiant temperature asymmetry is analysed to 1.1 m off the ground for standing and 0.6 m for seating conditions, and must be kept under the following limits (Table 3).


**Table 3.** Values of asymmetric radiation for a PPD ≤10%

112 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

The degree of user acceptance of the environmental conditions of the premises is a function of their air quality, which includes higrothermal comfort and adequate pollution levels.

There is enough information about the influence of higrothermal conditions on the predicted percentage of dissatisfied (PPD) in a room (Fanger, 1993a). The expressions that

The *PPD* as a function of temperature gradient is given by the equation (ASHRAE, 2009a):

*<sup>e</sup>* <sup>−</sup> <sup>=</sup> +

The changes in the *PPD* by the synergistic effects of air velocity, temperature and

where: *Ta* is the air temperature (ºC), *v* is the air velocity (m/s) and *Tu* is the turbulence intensity (%), considering that the air turbulence intensity in a point is defined by the

*v*

*v* σ

where *σv* is the standard deviation and *v* is the mean air velocity of a random sample of

The percentage of dissatisfied by warm or cold floor can be deduced through the following expression (BS EN ISO 7730:2005, 2006c), that relates the *PPD* to the floor temperature *tf*.

Asymmetric radiation from warm or cold surfaces, created by high lighting levels, due to large glazed surfaces or direct sunlight can reduce thermal acceptability of the spaces (BS EN ISO 7730:2005, 2006d). Radiant temperature asymmetry is analysed to 1.1 m off the ground for standing and 0.6 m for seating conditions, and must be kept under the following

*PPD e* −+ −

100 94 *f f t t*

( ) <sup>2</sup> 1.387 0.118· 0.0025·

= − (11)

*uT*

( ) 5.76 0.856 100 1 *TG*

*PPD* = (34 - *Ta*)·(*v* – 0.05)0.62· (0.37·*v·Tu* + 3.14) (9)

= (10)

(8)

relate the PPD with different causes of higrothermal discomfort are the following:

*PPD*

b. Synergistic effect on PPD of air velocity, temperature and turbulence

turbulence, are defined by the expression (BS EN ISO 7730:2005, 2006a, 2006b):

where *TG* is the vertical thermal gradient between head and feet.

**2.5. Reduce the causes of discomfort** 

a. PPD due to vertical temperature gradient

c. PPD due to inadequate floor temperature

d. PPD due to asymmetric thermal radiation

equation:

velocities.

limits (Table 3).

With respect to air quality, the effect on the PDP of the contaminants perceived by the sense of smell has also been extensively discussed (Fanger, 1993b), but it does not when it comes to pollutants that are not detected by humans, for it is unknown its influence on the PPD and how affects the workers productivity.

e. PPD due to human detected pollutants

Fanger established how the percentage of people dissatisfied is influenced by the presence of pollutants, provided that they can be perceivable. For low concentrations (usual when working indoors) the relationship is linear. For low concentrations (usual working indoors) the relationship is linear.

Of pollutants detected by humans, one of the most common is carbon dioxide (CO2). The relationship between the *PPD* and the concentration of CO2 in the air can be determined through the expression (Fanger, 1993c):

$$PPD = \Im95 \, e^{\left[-15.15 \cdot \left(\text{ $\mathbb{C}$ } - 350\right)^{-0.25}\right]} \tag{12}$$

where *C* is the carbon dioxide CO2 concentration in ppm.

From the previous expression it can be deduced:


Under the hypothesis that contaminants not perceived could be studied under the aforementioned theoretical frame, so that their effect on humans could be brought together and their contribution to the PPD could be analyzed, Gomez (2009) has developed a method for quickly and easily determine the PPD for any room with any type of pollution (detected or not by humans). It is based upon the concentrations of pollutants prescribed by international health agencies, for which there is adopted the type of curve derived by Fanger for CO2 as an average value.

ACGIH (1999) and the existing standards on health and hygiene at work suggest the maximum allowable concentration of pollutants for different periods of time TLV (Threshold Limit Values). According to them, being exposed to a specific concentration of pollutants for 8 hours a day leads to a *PPD* of 100%. This concentration is known as the TLWA value.

By analogy with the case of CO2, whatever the type of pollutant studied, it can be constructed a function that relates its concentration with the caused *PPD*, since three points of it are known. Firstly, the curve passes through the origin. Secondly, it is also known the concentration which produces a 100% *PPD*. And finally, according to the experience for CO2, a *PPD* of 15% is reached with a concentration 11.76 times lower than the one which causes a *PPD* of 100%.

High Efficiency Mix Energy System Design with Low Carbon Footprint for Wide-Open Workshops 115

**Figure 2.** PPD due to combined action of higrothermal and health factors

The effectiveness of air renewal provided by the ventilation system is the essential parameter for the design of an air diffusion system. Effectiveness, rv (-), of a ventilation

*extr imp*

<sup>−</sup> <sup>=</sup> − (13)

*C C*

*C C*

with Cimp, contaminant concentration in supply air; Cext, contaminant concentration in

As it is shown in Table 4, among the systems used in air diffusion, , used in its early stages only in industrial premises (Baturin, 1972), shows as the air diffusion system that eliminate sources of contamination most efficiently, especially when air is driven at a lower temperature than the indoor air. This always occurs during cooling period, and also with

**Mixed mode Displacement mode** 

**ΔT rv ΔT rv** < 0 0.9 … 1 < 0 1.2 … 1.4 0 … 2 0.9 0 … 2 0.7 … 0.9 2 … 5 0.8 > 2 0.2 … 0.7

**Table 4.** Ventilation effectiveness dependence on air diffusion mode and difference of temperatures

With this systems air is supplied at very low velocity near the floor, within the occupied zone (Nielsen, 1993). This is, then, the coolest zone of the room, and forces a profile of temperature and pollutant concentration which vertically increases up to the roof (Figure 3),

*zona imp*

*v*

*r*

exhaust air; y Czona, contaminant concentration in the occupied zone.

**2.6. Election of an adequate diffusion air system** 

system is defined with the expression:

ventilation air in winter.

where the return takes place.

> 5 0.4 … 0.7

The error made with this assumption depends on the shape of the curve that relates *PPD* to concentration of each pollutant. In the case of the substances for which ACGIH provides the permissible limit for TLV-STEL (concentration at which users may be exposed continuously for 15 minutes without chronic or irreversible damage) and TLV- TLWA, they are related within a range from 1.5 up to 6.

NIOSH REL provides a value of 6 for the relation between permissible concentrations of CO2 for 8 hours and 15 minutes (ASHRAE, 2009b), while German list of MAK (Maximale Arbeitsplatzkonzentration) values (Deutsche Forschungsgemeinschaft, 2007) gives a relation value of 2.

It then follows that it can be adopted for the rest of pollutants the type of curve derived by Fanger for CO2 without making significant errors. It is expected that further investigations in this field could provide new data. For permanent flow, as the *PPD* of the interior of the premises takes low values, it is only used the straight part of the curve, in which for a *PPD* of 15%, concentration is 1/11.76 of that corresponding to 100%.

Through "in situ" measurements, or prediction by means of CFD models, of the concentration of different contaminants for specific points of the room, it can be deduced the PPD achieved under any circumstance. This method can be used whatever the air diffusion system considered, though in the next section it will be discussed the advantages and drawbacks of the usual systems.

When it comes to know the indoor air quality of a room it is essential to determine the influence of ventilation air flow rate in the PPD. Considering that its effect on higrothermal conditions, and eventually on the PPD achieved, is opposite to pollutants concentration (Figure 2), the authors Castejon et al. (2011) and Galvez-Huerta et al. (2012) have recently studied the problem of settling the ventilation rate that minimizes the predicted percentage of dissatisfied in a room. The final aim is that this result in energy savings, for it minimizes the ventilation air flow rate.

In wide-open buildings, draughts and unwanted air currents can be a constant feature when the door opens automatically, affecting higrothermal comfort. At the same time, discomfort if there is a significant difference in temperature of their feet and their head can be fairly controlled through an air diffusion system.

With respect to the existing sources of contaminants, the rate of air change will depend on the carbon dioxide and monoxide emissions.

**Figure 2.** PPD due to combined action of higrothermal and health factors

#### **2.6. Election of an adequate diffusion air system**

114 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

causes a *PPD* of 100%.

value of 2.

within a range from 1.5 up to 6.

drawbacks of the usual systems.

the ventilation air flow rate.

controlled through an air diffusion system.

the carbon dioxide and monoxide emissions.

By analogy with the case of CO2, whatever the type of pollutant studied, it can be constructed a function that relates its concentration with the caused *PPD*, since three points of it are known. Firstly, the curve passes through the origin. Secondly, it is also known the concentration which produces a 100% *PPD*. And finally, according to the experience for CO2, a *PPD* of 15% is reached with a concentration 11.76 times lower than the one which

The error made with this assumption depends on the shape of the curve that relates *PPD* to concentration of each pollutant. In the case of the substances for which ACGIH provides the permissible limit for TLV-STEL (concentration at which users may be exposed continuously for 15 minutes without chronic or irreversible damage) and TLV- TLWA, they are related

NIOSH REL provides a value of 6 for the relation between permissible concentrations of CO2 for 8 hours and 15 minutes (ASHRAE, 2009b), while German list of MAK (Maximale Arbeitsplatzkonzentration) values (Deutsche Forschungsgemeinschaft, 2007) gives a relation

It then follows that it can be adopted for the rest of pollutants the type of curve derived by Fanger for CO2 without making significant errors. It is expected that further investigations in this field could provide new data. For permanent flow, as the *PPD* of the interior of the premises takes low values, it is only used the straight part of the curve, in which for a *PPD*

Through "in situ" measurements, or prediction by means of CFD models, of the concentration of different contaminants for specific points of the room, it can be deduced the PPD achieved under any circumstance. This method can be used whatever the air diffusion system considered, though in the next section it will be discussed the advantages and

When it comes to know the indoor air quality of a room it is essential to determine the influence of ventilation air flow rate in the PPD. Considering that its effect on higrothermal conditions, and eventually on the PPD achieved, is opposite to pollutants concentration (Figure 2), the authors Castejon et al. (2011) and Galvez-Huerta et al. (2012) have recently studied the problem of settling the ventilation rate that minimizes the predicted percentage of dissatisfied in a room. The final aim is that this result in energy savings, for it minimizes

In wide-open buildings, draughts and unwanted air currents can be a constant feature when the door opens automatically, affecting higrothermal comfort. At the same time, discomfort if there is a significant difference in temperature of their feet and their head can be fairly

With respect to the existing sources of contaminants, the rate of air change will depend on

of 15%, concentration is 1/11.76 of that corresponding to 100%.

The effectiveness of air renewal provided by the ventilation system is the essential parameter for the design of an air diffusion system. Effectiveness, rv (-), of a ventilation system is defined with the expression:

$$r\_v = \frac{\mathbf{C}\_{extr} - \mathbf{C}\_{imp}}{\mathbf{C}\_{zom} - \mathbf{C}\_{imp}} \tag{13}$$

with Cimp, contaminant concentration in supply air; Cext, contaminant concentration in exhaust air; y Czona, contaminant concentration in the occupied zone.

As it is shown in Table 4, among the systems used in air diffusion, , used in its early stages only in industrial premises (Baturin, 1972), shows as the air diffusion system that eliminate sources of contamination most efficiently, especially when air is driven at a lower temperature than the indoor air. This always occurs during cooling period, and also with ventilation air in winter.


**Table 4.** Ventilation effectiveness dependence on air diffusion mode and difference of temperatures

With this systems air is supplied at very low velocity near the floor, within the occupied zone (Nielsen, 1993). This is, then, the coolest zone of the room, and forces a profile of temperature and pollutant concentration which vertically increases up to the roof (Figure 3), where the return takes place.

When the emission of pollutants is uniformly distributed, as in places where human presence is dominant, the system design is based on the control of the temperature gradient (Mundt, 1995) to maintain comfort conditions in the occupied area: Namely, air temperature at 1.80 m high, maximum air velocity in the occupied zone, radiant asymmetry from the ceiling, walls and floor, and maximum temperature difference between the head and feet.

High Efficiency Mix Energy System Design with Low Carbon Footprint for Wide-Open Workshops 117

Consider that, with an average occupancy of the building of 5 m2/person, the Spanish mandatory ventilation air flow rate, supplied at 10ºC less than indoor air, can met a cooling load of 30 W/m2. This may seem certainly low, but is close to the expected for the coming

In the case of partially open workshops, the primary air handling unit can guarantee the quality of air in the building with a reasonable low energy consumption (for they supply air with indoor conditions) only if climate separation in openings has been previously solved. When extreme winter conditions have to be faced, primary air conditioning can be supplemented with water terminal units in the weakest points of the perimeter, the doors. In this sense, the placement of an air curtain in each opening can accomplish both tasks, reducing air movement within the premises to the necessary to remove contaminants.

The Spanish legislation (R.D. 1027/2007, 2007) makes it mandatory for air conditioning systems of a certain size the following specific strategies for energy recovery, framed in a

• Free cooling by outside air, which applies to constant volume or VAV air conditioning

• Recovery of heat from ventilation air with adiabatic cooling of the extract air. This requirement is applicable to buildings in which the exhaust air flow is greater than 0.5 m3/s. It also establishes minimum efficiency of recovery of sensible heat depending on the air flow rate and the number of hours the system is operating throughout the year

≤2,000 40 44 47 55 60 > 2,000...4,000 44 47 52 58 64 > 4,000...6,000 47 50 55 64 70 > 6,000 50 55 60 70 75

In the previous section Dedicated Outdoor Air Systems (DOAS) have been identified as the most appropriate systems to ensure adequate air quality in rooms with high concentrations

For this type of systems, energy recovery strategies should focus on the recovery of sensible

Tsalida = Text – η (Text – Tlocal) (14)

> 0.5...1.5 > 1.5...3.0 > 3.0...6.0 > 6.0...12 >12 % % % % %

**2.8. Maximize the cost-effectiveness of heat recovery equipment** 

**Annual operating hours Outdoor air flow rate (m3/s)**

**Table 5.** Minimum efficiency of sensible heat recovery system, according to I.T. 1.2.4.5.2

systems with rated cooling output greater than 70 kW.

more general requirement of energy efficiency:

heat, where the following relationship is satisfied:

with η (-), efficiency of sensible heat recovery, according to Table 4.

(Table 5).

of pollutants.

years.

**Figure 3.** Temperature and concentration of contaminants gradient

When, on the contrary, the emission of pollutants is concentrated, as in areas that have car engines running, in this case the cycle of air change will be greater than in those areas where less strenuous work is carried out. This allows to reduce the ventilation air to the minimum required, thus saving energy.

### **2.7. Use of Dedicated Outdoor Systems (DOAS)**

As some authors (Mumma, 2001) have expressed, a new paradigm in the design of HVAC systems is in its early stages. Amidst its requirements, the more remarkable are: separating outdoor air from the air conditioning system to ensure adequate ventilation and the use of energy recovery strategies.

Despite the fact that the new systems can also handle the space latent load and part of its sensible load, the transition from mixed ventilation systems to DOAS always compels to use a second air conditioning system (be it passive or active beams, water or direct expansion fan coils, ceiling cooling panels, radiant floors and thermoactive surfaces). This drawback of having duplicated systems is usually compensated with the use of high efficiency terminal units, which run on low temperature water, closer to the indoor air conditions.

However, it is usually forgotten that the present way towards more efficient buildings, with the commissioning Nearly Zero Energy Building by 2020 (Directive 2010/31/EU, 2010), has produced substantial improvements in the envelope and lightning systems, which have consequently reduced their contribution to cooling loads. But these improvements, while reducing consumption, also involve a decrease of the thermal load met by the coils. Consider that, with an average occupancy of the building of 5 m2/person, the Spanish mandatory ventilation air flow rate, supplied at 10ºC less than indoor air, can met a cooling load of 30 W/m2. This may seem certainly low, but is close to the expected for the coming years.

In the case of partially open workshops, the primary air handling unit can guarantee the quality of air in the building with a reasonable low energy consumption (for they supply air with indoor conditions) only if climate separation in openings has been previously solved. When extreme winter conditions have to be faced, primary air conditioning can be supplemented with water terminal units in the weakest points of the perimeter, the doors. In this sense, the placement of an air curtain in each opening can accomplish both tasks, reducing air movement within the premises to the necessary to remove contaminants.

#### **2.8. Maximize the cost-effectiveness of heat recovery equipment**

116 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

**Figure 3.** Temperature and concentration of contaminants gradient

**2.7. Use of Dedicated Outdoor Systems (DOAS)** 

required, thus saving energy.

energy recovery strategies.

When the emission of pollutants is uniformly distributed, as in places where human presence is dominant, the system design is based on the control of the temperature gradient (Mundt, 1995) to maintain comfort conditions in the occupied area: Namely, air temperature at 1.80 m high, maximum air velocity in the occupied zone, radiant asymmetry from the ceiling, walls and floor, and maximum temperature difference between the head and feet.

When, on the contrary, the emission of pollutants is concentrated, as in areas that have car engines running, in this case the cycle of air change will be greater than in those areas where less strenuous work is carried out. This allows to reduce the ventilation air to the minimum

As some authors (Mumma, 2001) have expressed, a new paradigm in the design of HVAC systems is in its early stages. Amidst its requirements, the more remarkable are: separating outdoor air from the air conditioning system to ensure adequate ventilation and the use of

Despite the fact that the new systems can also handle the space latent load and part of its sensible load, the transition from mixed ventilation systems to DOAS always compels to use a second air conditioning system (be it passive or active beams, water or direct expansion fan coils, ceiling cooling panels, radiant floors and thermoactive surfaces). This drawback of having duplicated systems is usually compensated with the use of high efficiency terminal

However, it is usually forgotten that the present way towards more efficient buildings, with the commissioning Nearly Zero Energy Building by 2020 (Directive 2010/31/EU, 2010), has produced substantial improvements in the envelope and lightning systems, which have consequently reduced their contribution to cooling loads. But these improvements, while reducing consumption, also involve a decrease of the thermal load met by the coils.

units, which run on low temperature water, closer to the indoor air conditions.

The Spanish legislation (R.D. 1027/2007, 2007) makes it mandatory for air conditioning systems of a certain size the following specific strategies for energy recovery, framed in a more general requirement of energy efficiency:



**Table 5.** Minimum efficiency of sensible heat recovery system, according to I.T. 1.2.4.5.2

In the previous section Dedicated Outdoor Air Systems (DOAS) have been identified as the most appropriate systems to ensure adequate air quality in rooms with high concentrations of pollutants.

For this type of systems, energy recovery strategies should focus on the recovery of sensible heat, where the following relationship is satisfied:

$$\mathbf{T}\_{\text{salda}} = \mathbf{T}\_{\text{ext}} - \mathbf{r}\_{\parallel} \left(\mathbf{T}\_{\text{ext}} - \mathbf{T}\_{\text{local}}\right) \tag{14}$$

with η (-), efficiency of sensible heat recovery, according to Table 4.

The temperature of the exhaust air, Texp, can be reduced by means of an adiabatic cooling process, in which the air supplied to the coils is cooled down to the temperature Tadiab, that can be calculated with the expression:

$$\mathbf{T}\_{\text{adiabatic}} \mathbf{T}\_{\text{local}} - \varepsilon \left(\mathbf{T}\_{\text{local}} - \mathbf{T}\_{\text{h}}\right) \tag{15}$$

High Efficiency Mix Energy System Design with Low Carbon Footprint for Wide-Open Workshops 119

**Figure 4.** Variable outlet width positions in an air curtain

**Figure 5.** Air curtain strength and heating control possibilities

**3.2. Heat production** 

three-way valve (Figure 6).

a. Conventional energy contribution

As an additional advantage, when used as heating terminal units they run on low temperature water, making them particularly suitable for being used with condensing boilers and solar thermal production. These options are discussed in the next two sections.

Air curtains and air handling unit heating coils run on low temperature hot water. Under such circumstances, thermal production can be provided by a solar thermal system. For the auxiliary energy supply a condensing boiler modular gas is projected. As with any solar installation, the energy collected is transferred to storage tanks, also connected to the boiler that takes charge of heating water when solar coverage falls. The control of the group of curtains and the primary air handling unit is done by varying the water flow rate with a

Condensation gas boilers use the heat content of vapor from the combustion, which is transferred to the heating system. As the heat conversion efficiency of the boiler is referred to the fuel net calorific value, performance values are reached greater than unity. As condensation inside the boiler begins when flue gases drop to about 54ºC, the boilers are

with ε (-), efficiency the evaporative cooling. As it is a function of the pad geometry and the air flow rate, it is difficult to establish a reliable average value.

The capacity for heat recovery in the primary air handling unit significantly increases when using a displacement air diffusion system. Because of extracting air near the roof, where maximum temperatures are reached, the system has a great potential for heat recovery in winter, but also in summer, when adiabatic cooling processes can be used, the more effective the higher the extract air temperature.
