Environmental and Water-Use Efficiency of Indirect Evaporative Coolers in Southern Europe ^†

Abstract

Heating, ventilation, and air-conditioning (HVAC) systems are responsible for about 20% of energy consumption in buildings. In terms of energy saving, evaporative cooling technology presents an interesting solution to conventional vapor compression systems. However, few studies have analyzed water-related indexes in indirect evaporative coolers (IECs). The main objective of this work was to evaluate the environmental impact and water-use efficiency of IECs in Southern Europe. Several models of performance indexes for the evaluated IEC system were developed via experimental tests. Based on energy simulations, the IEC system reached the highest values of annual SEER (7.6), $KPI$_C-W (0.28 kWh/L), and $KPI$_C-CO2 (16.2 kWh/kg CO₂) for Lampedusa weather conditions. These results show that the IEC system for hot climate zones reached the highest environmental and water-use efficiency values.

1. Introduction

According to several studies conducted in the last decade, heating, ventilation, and air-conditioning (HVAC) systems are responsible for about 20% of energy consumption in buildings [1]. Evaporative cooling technology presents an interesting solution to conventional cooling technologies based on vapor compression systems. Most of the experimental studies of indirect evaporative coolers (IECs) focused on the analysis of energy performance. However, few studies have analyzed water-related indexes in this type of air-cooling system. In a previous study, a methodology to determine the water-use efficiency of evaporative coolers was developed [2]. Another work showed high values of water-use efficiency for IEC technology and combined systems with IECs [3].

The main objective of this work was to evaluate the environmental impact and water-use efficiency of indirect evaporative coolers in different countries in Southern Europe.

2. Materials and Methods

2.1. Experimental Setup

An indirect evaporative cooler (IEC) was experimentally studied in the present work. The IEC system was mainly composed of a counterflow heat exchanger, a process fan, a gross 60% filter, and an ePM₁ 65% filter. The inlet air conditions were adjusted with an AHU, as shown in Figure 1.

Figure 1. Experimental configuration for the study of IEC.

The evaluated IEC system worked with a single inlet air stream (100% outdoor air), IA, which was divided into two air streams: exhaust air (EA) and outlet air (OA). This air-cooling system consisted of alternative wet and dry channels separated by thin plates. The inlet air flow was cooled without increasing its humidity ratio, and the exhaust air flow was humidified and then exhausted outside. The most important characteristics of this IEC are shown in Table 1.

Table 1. Main characteristics of evaluated IEC system.

Parameter	Value	Unit
Nominal cooling capacity	18	kW
Nominal inlet air flow rate	5000	m3 h−1
$\mathrm{Exhaust}\mathrm{air}\mathrm{ratio}(R_{EX}={\dot{V}}_{EA}/{\dot{V}}_{IA})$	0.45	-
Nominal power consumption	1.5	kW
Maximum water consumption	44	l h−1

2.2. Description of IEC Evaluation

Each experimental test was carried out under different working conditions during a steady-state period of thirty minutes. The experimental tests were used to obtain the models of the output parameters: cooling capacity ($\dot{Q}$_cooling) energy consumption ($\dot{W}$_cons), and water consumption ($\dot{V}$_W). Different values of inlet air temperature (T_IA), inlet air humidity ratio (ω_IA), inlet volumetric air flow ($\dot{V}$_IA), and exhaust air ratio (R_EX) were considered to develop the mathematical models; see Table 2.

Table 2. Summary of experimental tests in IEC system.

Test	TIA (°C)	ωIA (g/kg)	${\dot{\mathit{V}}}_{\mathit{I}\mathit{A}}$(m3/h)	REX (-)	Test	TIA (°C)	ωIA (g/kg)	${\dot{\mathit{V}}}_{\mathit{I}\mathit{A}}$(m3/h)	REX (-)
N1	32	11	3000	0.3	N6	40	11	3700	0.7
N2	32	8	3000	0.5	N7	32	11	4500	0.3
N3	32	11	3000	0.7	N8	32	14	4500	0.5
N4	40	11	3700	0.3	N9	32	11	4500	0.7
N5	40	8	3700	0.5

Second-order polynomial equations were used to obtain the relationship between the input parameters and the output parameters; see Equation (1).

$$\widehat{Y}=b_0+{\displaystyle {\sum }_{i=1}^k}{b}_i\cdot X_{\dot{i}}+{\displaystyle {\sum }_{i=1}^k}{b}_{ii}\cdot X_{\mathrm{i}}^2+{\displaystyle {\sum }_{i=1}^{k=1}}{\displaystyle {\sum }_{i=2;j>i}^k}{b}_{ij}·X_{\dot{i}}·X_j,$$

(1)

where $\widehat{Y}$ is the estimated output value; X is the input parameters; b₀ is the average response in each model; and b_i, b_ii, and b_ij are the estimated parameters of the linear, quadratic, and second-order terms, respectively.

However, energy simulations were carried out to compare the environmental impact of the IEC system under different climate zones in Southern Europe. TRNSYS software was used in the energy simulations. It should be noted that the annual cooling periods and CO₂ emission factors considered, F_CO2, for each city were taken from a recently published article [4]. The values of the inlet volumetric air flow rate ($\dot{V}$_IA) and R_EX were constant for the energy simulations; see Table 1.

2.3. IEC Evaluation Indexes

The IEC system was evaluated in terms of the output parameters of the experimental tests N1–N9 (see Equations (2)–(4)) and environmental impact indexes during annual cooling periods (see Equations (5)–(9)):

$$\mathrm{Cooling}\mathrm{capacity}:{\dot{Q}}_{\mathit{cooling}}={\rho }_{\mathit{air}}·{\dot{V}}_{\mathit{OA}}·(h_{\mathit{IA}}-h_{\mathit{OA}})\left[\mathrm{kW}\right]$$

(2)

$$\mathrm{Energy}\mathrm{consumption}:{\dot{W}}_{cons}={\dot{W}}_{Process\mathit{fan}}+{\dot{W}}_{Pump}\left[\mathrm{kW}\right]$$

(3)

$$\mathrm{Water}\mathrm{consumption}:{\dot{V}}_W=\mathrm{Measurement}\mathrm{of}\mathrm{IEC}\mathrm{water}\mathrm{consumption}\left[\mathrm{L/h}\right]$$

(4)

$$\mathrm{Annual}\mathrm{seasonal}\mathrm{energy}\mathrm{efficiency}\mathrm{ratio}:\mathit{SEER}=\frac{{{\displaystyle \sum }}^{} {\dot{Q}}_{cooling}}{{{\displaystyle \sum }}^{}{\dot{W}}_{cons}}[-]$$

(5)

$${\mathrm{CO}}_2\mathrm{emissions}:{\dot{E}}_{\mathit{CO}2}=F_{\mathit{CO}2}·{\displaystyle \sum }{\dot{W}}_{cons}[\mathrm{kg}{\mathrm{CO}}_2/\mathrm{year}]$$

(6)

$$\mathrm{Cooling}\mathrm{capacity}\mathrm{per}{\mathrm{CO}}_2\mathrm{emission}:{KPI}_{C-\mathit{CO}2}=\frac{{{\displaystyle \sum }}^{} {\dot{Q}}_{cooling}}{{{\displaystyle \sum }}^{}\dot{E}CO2}[\mathrm{kWh}/\mathrm{kg}{\mathrm{CO}}_2]$$

(7)

$$\mathrm{Cooling}\mathrm{capacity}\mathrm{per}\mathrm{unit}\mathrm{of}\mathrm{water}\mathrm{consumed}:{KPI}_{C-W}=\frac{{{\displaystyle \sum }}^{} {\dot{Q}}_{cooling}}{{{\displaystyle \sum }}^{} {\dot{V}}_W}[\mathrm{kWh}/\mathrm{L}]$$

(8)

$$\mathrm{Water}\mathrm{consumption}\mathrm{per}{\mathrm{CO}}_2\mathrm{emission}:{KPI}_{W-\mathit{CO}2}=\frac{{{\displaystyle \sum }}^{} {\dot{V}}_W}{{{\displaystyle \sum }}^{}\dot{E}CO2}[\mathrm{L}/\mathrm{kg}{\mathrm{CO}}_2]$$

(9)

3. Results and Discussion

3.1. Experimental Results

Cooling capacity, energy consumption, and water consumption were the output parameters evaluated in the IEC system. The results of these performance indexes for each experimental test are shown in Table 3. It can be observed that high values of $\dot{Q}$_cooling were achieved under the working conditions of tests N4 and N7. In both cases, the values of ω_IA and R_EX were 11 g/kg and 0.3, respectively. However, the value of $\dot{Q}$_cooling,N7 was higher than the value of $\dot{Q}$_cooling,N4 due to the higher inlet air flow rate and, thus, a higher outlet air flow rate. The lowest values of $\dot{W}$_cons and $\dot{V}$_W were reached when R_EX was 0.7.

Table 3. Experimental results of the output parameters for the studied IEC.

Test	$\dot{\mathit{Q}}$cooling (kW)	$\dot{\mathit{W}}$cons (kW)	$\dot{\mathit{V}}$W (L/h)	Test	$\dot{\mathit{Q}}$cooling (kW)	$\dot{\mathit{W}}$cons (kW)	$\dot{\mathit{V}}$W (L/h)
N1	17.5	0.377	14.6	N6	22.2	0.669	43.7
N2	16.7	0.399	21.3	N7	33.5	1.130	17.2
N3	13.3	0.376	37.8	N8	26.8	1.126	40.8
N4	30.0	0.692	17.3	N9	24.3	1.079	42.5
N5	28.3	0.703	23.2

The experimental results allowed for mathematical models of the output parameters to be obtained. The coefficients of determination (R²) for the $\dot{Q}$_cooling model, the $\dot{W}$_cons model, and the $\dot{V}$_W model of the evaluated IEC were greater than 0.987 for all output parameters.

3.2. Annual Results of Environmental Impact

The annual values of several indexes related to environmental impact were obtained (see Figure 2) according to Equations (5)–(9). The cooling period considered for each climate zone (Lampedusa, Seville, Thessaloniki, and Zagreb) was defined as the number of hours in which T_IA exceeded 18 °C.

Figure 2. Results of IEC annual environmental impact: (a) SEER,$\dot{E}$_CO2,$KPI$_C-CO2; (b) $KPI$_C-W,$KPI$_W-CO2.

It can be observed that, for all the selected cities, the results of $KPI$_C-W and $\dot{E}$_CO2 increased when the SEER value increased. The IEC system in Lampedusa weather conditions, with the longest cooling period, reached the highest values of SEER (7.6) and $KPI$_C-W (0.28 kWh/L) but also the highest value of $\dot{E}$_CO2 (3013 kg CO₂/year); see Figure 2. According to $KPI$_W-CO2, the results were similar for the Seville and Lampedusa climate zones. The IEC for Zagreb weather conditions showed the highest $KPI$_W-CO2 value (71.4 L/kg CO₂) due to low CO₂ emissions; see Figure 2. The $KPI$_C-CO2 values for Seville, Thessaloniki, and Zagreb were 1.8, 2.7, and 2.0 times lower than the $KPI$_C-CO2 value for the Lampedusa climate zone, respectively; see Figure 2.

4. Conclusions

In the present work, the environmental impact and water-use efficiency of indirect evaporative coolers (IECs) in Southern Europe were analyzed. Mathematical models of several performance indexes for the evaluated IEC system were developed via experimental tests, showing good agreement. Based on the annual results obtained, the IEC for Lampedusa weather conditions reached the highest values of SEER (7.6), $KPI$_C-W (0.28 kWh/L), and $KPI$_C-CO2 (16.2 kWh/kg CO₂). The IEC system for Zagreb weather conditions showed the highest value of $KPI$_W-CO2 (71.4 L/kg CO₂) due to the lowest value of $\dot{E}$_CO2 that IEC also showed (870.7 kg CO₂/year). These results show that the IEC system for hot climate zones reaches the highest environmental and water-use efficiency values.