**3. An outlook on high efficient air conditioning systems for wide-open workshops**

#### **3.1. Climate separation via air curtains**

Air curtains have the function of neutralizing outside air infiltration through the doors, reducing up to 90% heating and cooling thermal demand. In winter period, their use as hot water terminal unit makes also possible to meet the thermal load due to the infiltration rate not eliminated by the curtain. Reduction of latent loads demand is crucial in warm humid climates. Furthermore, they allow to control indoor environment regardless of external conditions.

To effectively carry out its function of climate separation, the curtains should maintain a proper discharge length whatever the external conditions of wind are. If the air curtain jet is too weak and the throw distance is short it does not prevent infiltrations. By contrast, excessive throw distance due to a strong jet can reduce efficiency by almost 50%. In this case, high velocity and turbulent flow make the air curtain partially mix with outside air.

The parameter that best characterizes the operation of a curtain is the momentum of jet, I0, that indicates the strength of the curtain. It is defined by the expression:

$$\mathbf{Io} = \mathbf{ç} \circ \mathbf{dc} \circ \mathbf{U} \circ \mathbf{2} \tag{16}$$

being d0 the outlet width and U0 the outlet velocity.

Modern air curtains vary the air flow driven maintaining a constant flow rate by adapting the geometry of the discharge outlet (Figure 4).

By choosing a relatively large outlet width, it can be achieved an optimum momentum, as needed to reach the floor, but with low velocity that keeps the flow in laminar regime.

Air curtain strength and heating can be controlled independently according to the needs (Figure 5).

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

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

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.

### **3.2. Heat production**

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

air flow rate, it is difficult to establish a reliable average value.

can be calculated with the expression:

effective the higher the extract air temperature.

**3.1. Climate separation via air curtains** 

**workshops** 

conditions.

(Figure 5).

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

Tadiab= Tlocal – ε (Tlocal – Th) (15)

with ε (-), efficiency the evaporative cooling. As it is a function of the pad geometry and the

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

**3. An outlook on high efficient air conditioning systems for wide-open** 

Air curtains have the function of neutralizing outside air infiltration through the doors, reducing up to 90% heating and cooling thermal demand. In winter period, their use as hot water terminal unit makes also possible to meet the thermal load due to the infiltration rate not eliminated by the curtain. Reduction of latent loads demand is crucial in warm humid climates. Furthermore, they allow to control indoor environment regardless of external

To effectively carry out its function of climate separation, the curtains should maintain a proper discharge length whatever the external conditions of wind are. If the air curtain jet is too weak and the throw distance is short it does not prevent infiltrations. By contrast, excessive throw distance due to a strong jet can reduce efficiency by almost 50%. In this case,

The parameter that best characterizes the operation of a curtain is the momentum of jet, I0,

I0 = ρ0·d0·U02 (16)

Modern air curtains vary the air flow driven maintaining a constant flow rate by adapting

By choosing a relatively large outlet width, it can be achieved an optimum momentum, as needed to reach the floor, but with low velocity that keeps the flow in laminar regime.

Air curtain strength and heating can be controlled independently according to the needs

high velocity and turbulent flow make the air curtain partially mix with outside air.

that indicates the strength of the curtain. It is defined by the expression:

being d0 the outlet width and U0 the outlet velocity.

the geometry of the discharge outlet (Figure 4).

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 three-way valve (Figure 6).

a. Conventional energy contribution

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

particularly suitable when the facility is operating at part load or when using terminal units operating at low temperature.

High Efficiency Mix Energy System Design with Low Carbon Footprint for Wide-Open Workshops 121

**Boiler type Reference** 

Normal gas boiler

Natural gas Natural gas Mains

*7,526 m3*

21,054

23,160 25,265 23,160

67,108 kWh

Direct electricity

electricity supply

> *150,695 kWh*

> > 2,317 m3

Modular gas condensation

35,459

0.280 0.649 0.204 *0.204 0.649* 

39,005 42,551 39,005 *223,394 56,735* 

42,165 110,761 39,434 *62,737 147,681* 

3,902 m3

providing supplementary source of energy heating the water demanded by the air

Electric boiler

Name TRISTAR General MODULEX General General

emersion heater

113,021 kWh

Total carbon emitted (kg) 11,806 71,884 **8,045** *12,798 95,845*  **Table 6.** Carbon emission levels for an ITV workshop in Madrid when energy demand is met only by

**Impact on Energy and Carbon emissions for energy supplied by the solar installation, rather than the boiler**  Boiler option number 1 2 3

Annual amount of fuel not consumed (kWh) 25,036 65,766 23,415

Amount of carbon emissions avoided (kg) 7,010 **42,682** 4,777

m3

boiler and condenser

> 4 m3

Boiler option number 1 2 3

Fuel source Diesel -C Electrical

curtains.

(kWh)

(kWh)

(kWh)

the system

the boiler

(kWh)

installation (kWh)

Diesel

Annual energy demand

Conversion factor for the carbon emitted from energy consumed (kgCO2/kWh)

consumption by the boiler

consumption from the fuel

Total energy consumed by

Annual energy savings provided by the solar

Annual energy demand not provided by boiler

Energy that was not consumed 2.5

**Table 7.** Impact of solar collectors for an ITV workshop in Madrid

Annual energy

Annual energy

**Figure 6.** Proposed heating system for wide-open workshops (diagram)

The effect of this high efficiency boilers, with seasonal efficiency of 0.97, on the overall efficiency of the heating system as opposed to normal gas or electric boilers (Table 6) has been assessed in a recent work by the authors (Gil-Lopez et al., 2011).

b. Renewable energy contribution

The savings achieved with the use of solar energy in thermal plants is sufficiently well known. However, when considering the impact on the carbon emissions avoided by the provision of supplementary carbon free energy by solar heating, an interesting result is obtained: the largest amount of carbon emissions avoided occurs when the solar installation provides supplementary energy to less efficient energy sources (Table 7).

This problem, similar to that experienced when using economic and investment indicators that favour consumption and not savings has been fully studied by the authors (Gil-Lopez et al., 2011). The question that arises is how can be assessed the impact of the solar installation in a way that reflects the value of the savings being made and not the energy being consumed. This can be easily seen by the impact the solar installation has on the energy certification (Table 8a and b).

As it was expected, the electric emersion heater boiler has the lowest certification value of G, the diesel and condenser option a C level, and the modular gas condensation boiler a certification rating of B. When the same calculations are conducted but with the solar installation providing supplementary power during part of the year, the certification ratings for the air curtains powered by hot water both receive a value rating of A, whereas that for the electric emersion heater boiler, although it has an improved indicator value, retains its G rating certification. Therefore, to obtain an A rating energy certification the air curtains need to be powered by hot water supplied through a combination of either diesel with condenser unit or a modular gas condensation boiler, with a solar installation providing supplementary source of energy heating the water demanded by the air curtains.

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

**Figure 6.** Proposed heating system for wide-open workshops (diagram)

been assessed in a recent work by the authors (Gil-Lopez et al., 2011).

provides supplementary energy to less efficient energy sources (Table 7).

operating at low temperature.

b. Renewable energy contribution

certification (Table 8a and b).

particularly suitable when the facility is operating at part load or when using terminal units

The effect of this high efficiency boilers, with seasonal efficiency of 0.97, on the overall efficiency of the heating system as opposed to normal gas or electric boilers (Table 6) has

The savings achieved with the use of solar energy in thermal plants is sufficiently well known. However, when considering the impact on the carbon emissions avoided by the provision of supplementary carbon free energy by solar heating, an interesting result is obtained: the largest amount of carbon emissions avoided occurs when the solar installation

This problem, similar to that experienced when using economic and investment indicators that favour consumption and not savings has been fully studied by the authors (Gil-Lopez et al., 2011). The question that arises is how can be assessed the impact of the solar installation in a way that reflects the value of the savings being made and not the energy being consumed. This can be easily seen by the impact the solar installation has on the energy

As it was expected, the electric emersion heater boiler has the lowest certification value of G, the diesel and condenser option a C level, and the modular gas condensation boiler a certification rating of B. When the same calculations are conducted but with the solar installation providing supplementary power during part of the year, the certification ratings for the air curtains powered by hot water both receive a value rating of A, whereas that for the electric emersion heater boiler, although it has an improved indicator value, retains its G rating certification. Therefore, to obtain an A rating energy certification the air curtains need to be powered by hot water supplied through a combination of either diesel with condenser unit or a modular gas condensation boiler, with a solar installation


**Table 6.** Carbon emission levels for an ITV workshop in Madrid when energy demand is met only by the boiler


**Table 7.** Impact of solar collectors for an ITV workshop in Madrid


High Efficiency Mix Energy System Design with Low Carbon Footprint for Wide-Open Workshops 123

*T*0.1 = *Ts* + θ0.1 (*TR* - *TS*) (17)

*Tf* = *T*0.1 - *αc,f* / *hi* (19)

<sup>−</sup> = + − (20)

*T T grad T* <sup>−</sup> <sup>=</sup> (21)

(18)

conditioning system efficiency, as air can be driven to higher temperatures achieving the same degree of thermal comfort for the workers through a well calculated design for


, ,

 α

+ + 

1 1 <sup>1</sup> *<sup>s</sup> c f rc*

1

terminal air velocities and temperature.

a. The following variables are known: - Specific cooling load, Φ*r*, (W/m2).

c. Calculation of floor temperature, *Tf*

m height, *T*1.8, is obtained:

According to Skistad (1994), the design procedure is the following:

b. Resolution of air temperature value at 0.1 m height.

where θ0.1, dimensionless temperature, is obtained with Mundt law:

0.1

=

θ

with air at low velocity, it can be considered 1/*hi* = 0.13 m2K/W

as well as the corresponding temperature gradient:

*Q c*

ρ

α

in which *αc,f* is the convective heat exchange between air and floor, that usually adopts a value of 4.5 W/m2. With respect to radiant heat exchange between air and ceiling, *αr,c*, it is obtained, from the initially unknown ceiling temperature *Tc*, by means of an iterative process. Although usually is considered equal to *αc,f*, its value is far from being constant.

with *hi*, surface heat transfer coefficient between floor and air. For a horizontal heat flow

d. Assuming that the temperature gradient in the room is constant, air temperature at 1.8

1.8 0,1 1.7·

*T T*

0.1

*R R T T*

*H*

1.8 0.1 1.7

0.1

(a)


(b)

**Table 8.** a. Rating without solar energy for an ITV workshop in Madrid, b. Rating with solar energy for an ITV workshop in Madrid

#### **3.3. Displacement ventilation system**

It helps significantly reduce the level of pollution in the occupied zone and the required air flow compared with ventilation with mixing with turbulent flow. It also allows the integration of direct and indirect adiabatic cooling systems. Furthermore, it optimizes the air conditioning system efficiency, as air can be driven to higher temperatures achieving the same degree of thermal comfort for the workers through a well calculated design for terminal air velocities and temperature.

According to Skistad (1994), the design procedure is the following:


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

Diesel boiler

Quantity of carbon emitted without the solar

CO2 emissions per m2 of surface area of

Emission reference boiler: for natural

Emission reference boiler: for electrical

Quantity of carbon emitted with the solar

CO2 emissions per m2 of surface area of

**3.3. Displacement ventilation system** 

installation (kg)

gas

boiler

Natural Gas

installation (kg)

the building (kgCO2/m2)

an ITV workshop in Madrid

the building (kgCO2/m2)

Indicator value (kgCO2/m2)

**Certification value without the solar installation Main energy supply options** 

Name TRISTAR General MODULEX Boiler option number 1 2 3 Type of fuel Diesel-C Electricity Natural gas

> 7.90 kgCO2/m2

> 59.16 kgCO2/m2

Energy certification value **C G B**  (a)

**Certification value with the solar installation Main energy supply options** 

Name TRISTAR General MODULEX Boiler option number 1 2 3 Type of fuel Diesel-C Electricity Natural gas

Indicator value (kgCO2/m2) 0.37 2.28 0.26 Energy certification value A G A

(b) **Table 8.** a. Rating without solar energy for an ITV workshop in Madrid, b. Rating with solar energy for

It helps significantly reduce the level of pollution in the occupied zone and the required air flow compared with ventilation with mixing with turbulent flow. It also allows the integration of direct and indirect adiabatic cooling systems. Furthermore, it optimizes the air

Diesel boiler

and condenser

1,620 m2 7.29 44.37 4.97

and condenser Electric boiler

11,806 71,884 8,045

0.923 5.600 0.629

Electric boiler

4,796 29,202 3,268

1,620 m2 2.96 18.03 2.02

Modular gas condensation

Modular gas condensation


$$T\_{0.1} = T\_s + \Theta 0.1 \text{ ( $T$ -} $ ^\circ$ )\tag{17}$$

where θ0.1, dimensionless temperature, is obtained with Mundt law:

$$\theta\_{0,1} = \frac{1}{Q\_s c \rho \left(\frac{1}{\alpha\_{c,f}} + \frac{1}{\alpha\_{r,c}}\right) + 1} \tag{18}$$

in which *αc,f* is the convective heat exchange between air and floor, that usually adopts a value of 4.5 W/m2. With respect to radiant heat exchange between air and ceiling, *αr,c*, it is obtained, from the initially unknown ceiling temperature *Tc*, by means of an iterative process. Although usually is considered equal to *αc,f*, its value is far from being constant.

c. Calculation of floor temperature, *Tf*

$$T\_f = T\_{0.1} - \alpha\_{c,f} / \,\text{hi} \tag{19}$$

with *hi*, surface heat transfer coefficient between floor and air. For a horizontal heat flow with air at low velocity, it can be considered 1/*hi* = 0.13 m2K/W

d. Assuming that the temperature gradient in the room is constant, air temperature at 1.8 m height, *T*1.8, is obtained:

$$T\_{1.8} = T\_{0,1} + 1.7 \cdot \frac{T\_R - T\_{0.1}}{H\_R - 0.1} \tag{20}$$

as well as the corresponding temperature gradient:

$$\text{grad } T = \frac{T\_{1.8} - T\_{0.1}}{1.7} \tag{21}$$

With the obtained values, accomplishment of normative comfort conditions is tested, paying special attention to the temperature difference between head and feet (ASHRAE, 1992).

High Efficiency Mix Energy System Design with Low Carbon Footprint for Wide-Open Workshops 125

three following options: an air curtain system functioning as a climate separator (option 1); the previous system with a conventional air handling unit (option 2); and finally, a comprehensive air conditioning system of high efficiency that includes all aspects covered

The design of this system and the selection of components for the case study are due to Eng.

To analyze the performance of the technical systems for each of the proposed situations, a representative industrial building has been selected. Such is the case of a wide-open workshop, where technical assessments of roadworthiness are carried out on cars and trucks (ITV workshop). This type of buildings is of rectangular shape, with a surface area of 1620 m2 and a height of 7.5 m. It has no windows, but five entrances and exits, three of them at 5 m x 3 m and two at 5 m x 4.5 m. The exit doors are controlled to limit the flow of vehicles leaving the premises. These buildings, that outnumber 1,000 all around Spain, are usually located in remote areas just off main motorways. For the case study, an ITV workshop

Using a computer simulation of the building with the e-Quest program (Figure 10), a prediction of its performance for the aforementioned options has been obtained. E-Quest uses DOE2 engine to perform an hourly simulation of the building for a whole one-year time period. For each hour, heating and cooling loads are calculated. The performance of pumps, fans, boilers, chillers and every energy consuming equipment within the building is also simulated. Finally, the energy use of every end use, including lightning, is tabulated. Authors such as Crawley et al. (2008) provide a sound comparative study of the potential

Paul Gerard O'Donohoe, who has used technology developed by TAYRA S.L.

located in Carmona (Sevilla) was chosen. See attached plan in Figure 9.

**Figure 9.** ITV workshop in Carmona (Seville). Plan

offered by the most common simulation tools, DOE2 included.

by section 2 and 3 of this chapter.

## **3.4. Primary air conditioning unit**

Ventilation is provided by a primary air handling unit with indirect adiabatic cooling heat exchanger section (Figure 7). The unit has two water coils. The heat coil is supplied by the heat production system described in section 3.2. The cooling load is met by water from a compression chiller. An alternative system with a solar absorption chiller for cold production is also suggested, but not discussed in this chapter.

**Figure 7.** EQUAM adiabatic air handling unit

Air transformations in the air handling unit components are shown in the psycrometric chart (Figure 8), where O stands for outdoor air, S for air supplied to the coil, R for room conditions and EX for exhaust air.

**Figure 8.** Psycrometric processes of indirect adiabatic cooling
