Exergy Analysis and Optimization of a Building Air ...

1 Copyright © 2014 by ASME

EXERGY ANALYSIS AND OPTIMIZATION OF A BUILDING AIR CONDITIONING SYSTEM IN TROPICAL CLIMATE

Ricardo Salazar Department of Mechanical and

Aerospace Engineering, University of Rome “Sapienza”

Rome, Italy

Enrico Sciubba Department of Mechanical and


Rome, Italy

Claudia Toro Department of Mechanical and


Rome, Italy

ABSTRACT

The space conditioning sector is one of the highest exergy consumers and least efficient from the point of view of primary-to-end-use matching. Exergy analysis can be considered as a reliable tool for analyzing and optimizing energy consumption related to building conditioning systems. The present study presents a comparative exergy analysis of the air conditioning system of the TOTAL S.A. offices located in Caracas, Venezuela to finally achieve a reduction of the global electric energy use of the considered building. Starting from the provided thermal cooling load, different possible cooling chains (primary-to-final energy conversion chain) are considered in order to locate the thermodynamically more efficient one from an exergetic point of view. The internal air handler unit, which provides for the cooled and dehumidified air to the building, is fed by the energy obtained from different possible converters of renewable energy primary sources. Specifically, solar and hybrid photovoltaic-thermal (PV/T) panels coupled with an absorption refrigeration machine and with an ejector refrigeration cycle are analyzed. The study that has been carried on leads to identify the most convenient matching between final use and primary sources allowing to substantially reduce the global non-renewable energy consumption of the considered building.

1. INTRODUCTION In the last decades energy use in the world has become a crucial point in everyday agenda. The forecasts for energy supplies shortage and excessive CO2 levels are of growingconcern. Most developed countries strongly rely on

fossil fuels, such as oil, natural gas, coal and nuclear fuels for fission. In the United States residential and commercial buildings use more energy than the transportation or industry sectors, accounting for nearly 40% of total U.S. energy use [11]. Growth in population, enhancement of buildings services and comfort levels, together with the rise in time spent inside buildings, have raised building energy use to the levels of transport and industry sectors. Energy used in HVAC (Heating, Ventilating and Air Conditioning) accounts for approximately 20% of the total energy consumption nowadays. In Venezuela, the electric energy used by residential and commercial-office building sector is 26% and 16% respectively, that is almost half the electricity use of the country. Only the air-conditioning system represents approximately 50% of the electricity use in residential and commercial buildings [2][16]. Air-conditioning systems are one of the highest exergy consumers and least efficient from the point of view of primary-to-end use matching, so an exergy analysis can be considered as a reliable tool for analyzing and optimizing the final energy use related to these systems [5]. The usual parameter used to identify the efficiency of a plant or a single energy conversion component, is the energy efficiency. However this efficiency, also known as first principle efficiency, has a defect: it does not account for the type (quality) of the energy flow, therefore solely the first principle efficiency is not enough to estimate the actual performances of both a component and a whole plant. For this reason the exergy efficiency, which is much more effective, was introduced. Exergy function makes a quality-weighted evaluation of energy fluxes entering or leaving a component, showing the degradation of energy occurring in it and its real efficiency. Only applying the exergy efficiency the designer would be able to compare properly different plant solutions or single

Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014

November 14-20, 2014, Montreal, Quebec, Canada

IMECE2014-36764


components and choose the best one from a thermodynamic point of view. This study has been carried out with the purpose of identify the most convenient matching between final use and primary sources to substantially reduce the final energy use of an office building located in Venezuela. Starting from the provided thermal cooling load, different possible cooling chains (primary-to-final energy conversion chain) are considered in order to locate the thermodynamically more efficient one from an exergy efficiency point of view. The most convenient air-conditioning plant has been identify by using the process simulator CAMEL-Pro™ [1] to simulate each one of the cooling chains, and compare them performance. 2. OBJECT OF THE STUDY The main purpose of this study is to get a reduction of the final energy use (in this case, the electric energy taken from the main power-grid) of the air conditioning system of the TOTAL S.A. offices located in Caracas, Venezuela reducing system dependence on non-renewable sources. A previous study made by Ankah and Del Portillo [2] demonstrates that there is an excess in the use of electric energy by the air-conditioning system implying an increase in bills and furthermore penalties made by the companies of electricity distribution. They computed the total thermal load of the office as well as the energy used by all the components of the air-conditioning system in order to compare the results with the national and international indicators, highlighting that the air-conditioning system uses twice the energy that it should, according to those indicators. As mentioned before, exergy analysis is a reliable tool to achieve an improvement and optimization of energy use, reducing the degree of unsustainability of the building. Exergy analysis has been applied to energy conversion systems since the early 1970s with the aim of encouraging the rational use of energy, which means in essence to strive for a better matching of the quality levels of the energy supply and demand. The air conditioning plant initially consisting of an AHU, Heat Pump and Cooling Tower will be modified by introducing solar energy as the main primary source, thus decreasing the dependence on non-renewable sources. The external subsystem that provides for the energy needed to cool the air will be represented by solar collectors and photovoltaic hybrid panels. An intermediate system will be responsible for the conversion of high-temperature energy into refrigeration power using electric energy as less as possible. Two different alternatives for this intermediate system will be considered, an ejector refrigeration cycle and an absorption refrigeration machine. 3. PLANT-LAYOUTS A cooling plant can be considered as a chain consisting of several distinct and connected systems. The plant

currently in operation presents an internal system, in this case an air handling unit (AHU) that blows conditioned air to the building. The intermediate conversion system is represented by a chiller supplying chilled water to the AHU and fed by an external subsystem consisting of a cooling tower; electrical energy is needed to make work both the chiller and the cooling tower. The main purpose of this study is to reduce the final energy use from non-renewable sources by optimizing the cooling chain, by implementing solar-driven refrigeration devices. Basically the external subsystem that provides the energy required by the AHU will be represented by solar collectors and photovoltaic hybrid panels. Based on new solar-driven refrigeration technologies [17]-[4], the Ejector Refrigeration Cycle (ERC) and the Absorption Refrigeration Machine (ARM), were selected as alternative components to be part of different cooling chains, with the purpose to find the most convenient matching between final use and primary sources. After having developed, implemented and validated thermodynamic models of the above mentioned components in CAMEL-Pro™, they were properly connected to each other to compose the five plant configurations listed below: CASE 1: AHU + Heat Pump + Cooling Tower (Current Plant); CASE 2: AHU + ERC + Solar collector; CASE 3: AHU + ARM + Solar collector; CASE 4: AHU + ARM + PVT hybrid solar panels; CASE 5: AHU + ERC + PVT hybrid panels.

CASE Internal

system Intermediate

system External system

1 AHU Heat Pump Cooling Tower

2 AHU ERC + Tank Solar

collectors

3 AHU ARM + Tank Solar

collectors

4 AHU ARM + Tank PVT

panels

5 AHU ERC + Tank PVT

panels Table 1 - Cooling chain subsystems Table 1 shows the devices included in each subsystem of the cooling chain for all the configuration cases, grouped in internal, intermediate and external systems. Inlet and outlet streams for each case are reported in Table 2.


CASE Inputs Outputs 1 Electrical power for AHU,

water pumps, heat pump and cooling tower

Refrigeration load of the building

2 Electrical power for AHU, water and refrigerant

pumps Solar irradiance


3 Electrical power for AHU and water pumps Solar irradiance


4 Electrical power for AHU

and water pumps Solar irradiance

Refrigeration load of the building PVT Electrical

power 5 Electrical power for AHU,

water and refrigerant pumps

Solar irradiance

Refrigeration load of the building PVT Electrical

power Table 2 - Input and output streams of simulated cases

3.1. Case 1 This configuration represents the plant currently in operation. Based on data taken from [2] the AHU and the Cooling Tower have been modeled and implemented on CAMEL-ProTM process simulator. Figure 1 shows the scheme of the air-conditioning system.

The external moist-air (stream #1 in Fig. 1) is cooled and dehumidified in the AHU and then blown through the office (stream #2). The AHU is fed by a cooling stream (stream #4) coming from the Heat Pump which uses as external cooling system the cooling tower. Each component uses an electric power input (streams 11,12,13,14 and 15).

3.2 Case 2 In this case the cooling chain consists of solar collectors coupled with an ejector refrigeration cycle ERC. This kind of systems generally uses waste or solar energy as a heat source at temperatures above 363K. The ERC consists of two loops, the power loop and the refrigeration loop and uses refrigerant R245fa as working fluid. Figure 2 shows the scheme of the plant. The main energy-source now is the solar irradiance to the solar collector while the electric power is used just by the AHU and pumps. An insulated tank is used to store the thermal energy of water, in order to dispose of hot water whenever is required by the system. High-pressure refrigerant evaporates in the generator (primary flow, stream #1), this is then mixed in the ejector with low-pressure vapor refrigerant (secondary flow, stream #2) coming from the cooling coil of the AHU. Water at ambient temperature (stream #18) condensates the refrigerant

mixture (stream #3). The primary flow (stream #5) is then pumped to the generator and the secondary flow (stream 6) is brought to a lower pressure in the throttling valve to restart the cycle.

3.3. Case3

A different solar-driven refrigeration technology was implemented for this configuration plant, the Lithium Bromide/Water Absorption Refrigeration Machine (ARM). This machine consists of four main internal components responsible for the conversion of thermal energy into refrigeration power; generator, condenser, evaporator and absorber. Figure 3 shows the plant-layout. This configuration plant is basically an AHU connected to the ARM which is then coupled with solar collectors. The solar collectors feed the insulated tank with hot water. The only components that use electric power are the pumps and the AHU, the main energy input is constituted by the solar irradiance to the panel. As can be seen from Figure 3, the ARM generator uses hot water (stream #7) and the evaporator chills the water (stream #2) that is then pumped to the cooling coil of the AHU. Heat rejection processes occur in the absorber and in the condenser, water at ambient temperature (streams #3 and #5) is used as a heat sink.

3.4. Cases 4 and 5 These two last configurations are practically the same as the cases 2 and 3, the difference lies in changing the solar panels by the PVT hybrid panels, obtaining an additional electric power output (stream #31, Figure 4 and stream #32, Figure 5). This electric power output represents an important advantage for the system making it more sustainable and less-dependent on the electric energy coming from non-renewable sources. 4. SIMULATIONS The results obtained from the simulation of the 5 cases described in the previous chapter are here presented. For each case one simulation has been performed using a fixed supply air temperature and humidity in the AHU, so this device works under the same conditions. Each configuration has an energy requirement and a performance different from the others related to the type of components operating in the cooling chain and their effectiveness.

4.1. Data input The parameters that have been set in the AHU, which are the same for all the cases, are shown in Table 3. These values have been taken from the study made by Ankah and Del Portillo [2].


Parameter Value Volumetric flow rate of air (m3/s) 1.3035

Inlet air Temperature (K) Inlet air relative humidity (%)

300 70

Outlet air Temperature (K) Outlet air relative humidity (%)

289 60

Chilled Water Inlet Temperature (K) 280 Chilled Water Outlet Temperature (K) 286

Fan load losses (Pa) 650 Fan motor efficiency (%) 80

Cooling coil efficiency (%) 90 Table 3 - AHU Data input

CASE 1: Current Plant (AHU + Heat Pump + Cooling Tower); The data inputs of the Cooling Tower are shown in Table 4

Parameter Value Inlet water temperature (K) 308 Outlet water Temperature (K) Fan load losses (Pa)

302 550

Inlet air temperature (K) Inlet relative humidity of air (%)

300 70

Outlet air temperature (K) 306 Outlet relative humidity of air (K) 95 Water pressure loss (%) 5

Table 4 - Cooling Tower parameters The operation parameters of the Heat Pump are presented in Table 5.

Parameter Value Hot side Inlet water temperature (K)

302

Outlet water Temperature (K) Cold side

308

Inlet chilled water temperature (K) Outlet chilled water temperature (K)

286 280

Heat Pump Compressor efficiency (%) 80 Heat Pump Mechanical efficiency (%) 98

Table 5 - Heat pump parameters CASE 2: AHU + ERC + Solar collector. Operative conditions and main parameters of the solar collectors and ejector refrigeration cycle are shown in Table 6.

Parameter Value Solar collector Solar irradiance (kW/m2) 0.570 Air temperature (K) 300 Water temperature (K) 383

Eff0 (-) 0.836 a1 (W/m2K) 0.790 a2 (W/m2K2) 0.790 Ejector refrigeration cycle Generator Temperature (K) 363 Generator Pressure (kPa) 980 Condensation Temperature (k) 298 Condensation Pressure (kPa) 149.5 Evaporation Temperature (K) 284 Evaporation Pressure (kPa) 87.06 Table 6 - Case 2: Solar collectors and ERC data input

CASE 3: AHU + ARM + Solar collector Solar collectors parameters are the same of the previous case (Table 6), Table 7 shows the data input of the ARM [6].

Parameter Value Generator Temperature (K) 363 Evaporator Temperature (K) 278 Condenser Temperature (K) 313 Absorber Temperature (K) 313

Heat exchanger effectiveness (%) 80 Refrigeration Load (kW) 70

Table 7 - ARM data input CASE 4&5: AHU + ARM/ERC + PVT hybrid solar panels. Cases 4 and 5 are basically the same as 2 and 3 but replacing the solar collectors by PVT hybrid panels. The parameters and working conditions of the rest of the components are the same. The main input data of the PVT [13] panels are shown in Table 8.

Parameter Description Value εg Glass cover radiation factor 0.05 εr Absorbing surface radiation

factor 0.1

αg Glass cover absorbing factor 0.05 αabs Absorbing plate absorbing

factor 0.75

τg Transmittivity from cover glass to absorbing surface

0.95

δ (m) Air –gap depth 0.02 PF (-) Cell packing factor 0.79 η0 (-) PV reference efficiency 0.15

Table 8 - Input parameters of the PVT panels in CAMEL


Figure 1 - Case 1: Current air-conditioning plant scheme in CAMEL-ProTM

Figure 2 - Case 2: Plant scheme in CAMEL-ProTM



Figure 4 Case 4: Plant scheme in CAMEL-ProTM



4.2 Simulation results CASE 1: Current Plant (AHU + Heat Pump + Cooling Tower); Tables 9 and 10 show the thermodynamic and physical properties of moist-air and water streams obtained from the process simulation in CAMEL-ProTM .

Id T p φ V h s [K] [kPa] [%] [m3/s] [kJ/kg] [kJ/kg K] 1 300 101.3 0.7 1.30 38.31 7.03 2 289 101.36 0.65 1.23 6.59 6.92 9 300 101.3 0.7 1.27 38.31 7.03

10 306 101.36 0.95 1.33 80.71 7.17 Table 9 - Moist air properties for Case 1 simulation

Id m p T h s

- [kg/s] [kPa] [K] [kJ/kg] [kJ/kg

K] 3 0.013 101.3 284 -66.98 0.17 4 2.55 105.3 280 -84.39 0.104 5 2.55 105.3 286 -59.23 0.19 6 2.49 103.2 308 33.44 0.51 7 2.49 98.03 302 8.36 0.42 8 0.023 103.3 298 -8.36 0.37 Table 10 - Water properties for Case 1 simulation

The identification numbers of each stream in Tables 9 and 10 are referred to Fig. 1 layout. Table 11 shows the electric power used by the different devices of the plant.

Component P [kW] Heat Pump 13.12

AHU 5.81 Cooling Tower 1.02

Water pump 0.021 Water pump 0.0061

Table 11 – Case 1 electric energy use CASE 2: AHU + ERC + Solar collector. Physical and thermodynamic properties of R245fa and water are reported in Table 12 and 13. The identification numbers of each stream correspond to those presented in Fig. 2.

Id m p T h s q

[kg/s] [kPa] [K] [kJ/kg

] [kJ/kg K] [kg/kg]

1 0.38 980 363 45.35 0.012 1 2 0.31 87.06 286 -11.79 -0.03 1 3 0.68 149.5 298 -2.58 -0.03 1 4 0.68 149.5 296 -196 -0.68 0


5 0.38 149.5 296 -196 -0.68 0

6 0.31 149.5 296 -196 -0.68 0

7 0.31 87.06 284 -196 -0.68 0.077 8 0.38 980 296 -195 -0.68 0

Table 12 - Properties of R245fa in Case 2

Id m p T h s [kg/s] [kPa] [K] [kJ/kg] [kJ/kg K]

12 2.14 105 373 305.18 1.31 13 2.14 101.85 363 263.07 1.19 14 2.14 101.85 373 305.17 1.31 15 2.17 151.9 383 347.44 1.42 16 2.17 151.9 373 305.84 1.31 17 2.17 155 373 305.85 1.31 18 6.33 101.3 293 -29.28 0.29 19 6.33 101.3 298 -8.36 0.37

Table 13 - Water properties in Case 2 A surface of 210 m2 for the solar collectors was obtained as a result of the simulation. This surface is the necessary to heat the amount of water required for the cycle to evaporate the refrigerant in the power loop of the ERC. Since there are 6 AHU in operation, a total surface of 1260 m2 should be used. The AHU uses the same amount of electrical energy as case 1 whereas the pumps use 0.3 kW so it can be neglected CASE 3: AHU + ARM + Solar collector Table 14 shows the water properties obtained from the process simulation. Each point of the cycle is identify in Fig. 3.

Id m p T h s [kg/s] [kPa] [K] [kJ/kg] [kJ/kg K]

1 2.55 102.9 280 -83.7 0.11 2 2.55 99.81 286 -58.6 0.19 3 3.27 105 293 -29.3 0.29 4 3.27 105 298 -8.4 0.37 5 3.74 101.3 293 -29.3 0.29 6 3.74 101.3 298 -7.7 0.37 7 2.02 105 368 284.7 1.25 8 2.02 100.8 358 242.7 1.13 9 2.02 100.8 368 284.7 1.25

10 2.02 104.86 373 305.8 1.31 11 2.02 104.86 363 263.7 1.19 12 2.02 107 363 263.71 1.19

Table 14 - Case 3: simulation results

In this Case the solar collectors’ surface required is equal to 200 m2, so approximately 114 kW of solar energy are used to heat the water and feed the ARM. Since there are 6 AHUs, the total surface needed by the three floors of the office is 1200 m2. CASE 4 and 5: AHU + ARM/ERC + PVT hybrid solar panels.

Cases 4 and 5 are exactly the same as cases 2 and 3 as well as the simulations results with the only difference that an electrical output is obtained from the internal conversion process in the PVT panels. Since PVT panels have different features than solar collectors, the plan surface required to satisfy the heat demand of the refrigeration cycles is also different. Case 4 which is the ARM coupled with the PVT panels needs a surface of 250 m2 per AHU and since the solar radiation is equal to 0.570kW/m2, the solar energy received by the panels is roughly 142 kW. The total surface covered when the 6 AHUs are in operation is 1500 m2. The electric power output is 8.7 kW each 250 m2 of surface, so the total electric power converted in 1500 m2 is roughly 52.2 kW which can be used to feed the same AHUs, pumps and auxiliaries. In Case 5 the ERC is connected to the PVT panels, the surface required to provide the necessary amount of thermal energy is equal to 270 m2, which represents 155 kW of solar energy captured by the panels, of course this is just for one AHU, when the 6 AHUs work together the total surface becomes 1620 m2. In this case the panels receive 155 kW each 270 m2 and 8.5kW are the result of converting solar energy into electric power. A total electric power of 51 kW is obtained when the 6 systems work together. 5. EXERGY ANALYSIS The introduction of exergy as a thermodynamic analysis tool can help to achieve the objective of reducing the degree of unsustainability of modern buildings. Exergy analysis has been applied to energy conversion systems since the early 1970s with the aim of encouraging the rational use of energy, which means in essence to strive for a better matching of the quality levels of the energy supply and demand [12]. Exergy analysis offers the potential of minimizing energy resource depletion. Exergy recognizes that the energy that is carried by substances can only be used down to the level that is given by the environment.

5.1 Exergy concepts

Exergy can be defined as the maximum theoretical work obtainable by a system when it is brought to a state of stable (possibly dynamic) equilibrium with the reference environment by means of ideally reversible transformations in which it exchanges heat only with the environment and only at T0. The origins of exergy are rooted in the Second Law of thermodynamics, which adopts entropy as the indicator of system irreversibility, thus providing a unique direction for spontaneous energy transformations that cannot be derived by the first law. For a detailed review of the literature about exergy fundamentals, see [9]. Unlike energy, exergy is not conserved: every process that is affected by energy degradation (entropy generation) destroys an


amount of the input exergy proportional to the total entropy production. Buildings use a great amount of energy but have the potential to consume small amounts of exergy so a great amount of exergy is destroyed. Most of their demand is for low- and medium temperature energy, such as space heating and cooling and domestic hot water. This demand is mostly supplied with electricity and natural gas or oil boilers, with energy quality factor of the source close to the unity. Exergy analysis and efficiency provide a very effective tool to improve the energy use for buildings. Exergy explains univocally the concept of energy quality. Reducing the exergy losses in either a system or a process and increasing its exergy efficiency means to use energy in a more rational way. High exergy overall efficiencies mean exploiting all the available exergy content and using energy in the most rational way[8]. Exergy analysis in buildings aims to reduce the exergy input and the consumption of exergy from non-renewable sources. An increase of the degree of sustainability of space conditioning systems can therefore only be attained by designing the overall system (including primary-to-final energy conversion) with higher exergy efficiency. A useful parameter to evaluate the exergy performance in a system is the Exergetic efficiency defined as the ratio between the exergy of the product and the required exergy input as shown in equation (1).

1 (1)

Exergy efficiency quantifies the amount of work extracted by a device out of the overall available, which would be exchanged in virtual, reversible conditions. As a consequence, it gives a potential of the degree of irreversibility that take place. The exergy destruction is represented as the difference between the exergy input and the exergy output, which in turn is the sum of the useful exergy output and the exergy losses -streams transferred to the environment that are not used by the process [3]. Exergy losses thus represent a physical flow of “unused” exergy into the surroundings, whereas exergy destruction indicates the annihilation of exergy within the system boundary due to irreversibility.

5.2. Exergy of moist-air

As described by Bejan [3], exergy or the maximum work performed by a current consisting of various chemical species that reaches thermal, mechanical and chemical equilibriums with the environment (T0, p0, µ0,1, ....., µ0,n) is given by: (2)

where , and denote the total molar exergy flow, the physical exergy, and the chemical exergy of the mixture, respectively. The physical exergy and the chemical exergy in molar basis are given by (3)

∑ μ μ , (4)

To obtain the total molar exergy of moist air stream, moist air has been assumed as a mixture of ideal gases composed of dry air and water vapor. Keeping that in mind and applying equation (2) to each component of the mixture, the following equation is obtained: μ μ ,

μ μ , (5)

and are the molar fractions of dry air and water vapor present in the mixture. Using the ideal gas model to evaluate the enthalpy, entropy, and chemical potential of the dry air, we have

(6)

ln ln (7)

μ μ , T ln,

(8)

Using again the ideal gas model, similar results can be obtained for water vapor. Combining equations (6)-(7) together with their expressions for water vapor into equation (5), we obtain

1 ln ln

ln,

ln,

(9)

Rewriting the equation (9) on a mass basis, we obtain the total exergy of moist air per kg of dry air as

1 ln 1 1.608 ln

1 1.608 ln.

.1.608 ln (10)

T, p and ω denote the temperature, pressure and humidity ratio of moist air, respectively. Humidity ratio is defined as the ratio of the mass of water vapor to the mass of dry air. The subscript (0) indicates properties evaluated in a dead state condition. Usually, the atmospheric condition (T0, p0, ω0) is selected as dead state condition.


5.3 Exergy analysis results On the basis of the theoretical concepts and plant design configurations explained above, the calculations of the exergy efficiency of the whole plants and the relative main components will be performed in this section. The original plant currently in operation in the office has been simulated in order to identify the components where the largest amount of exergy is destroyed and those which use a high-exergy source, this can be made by calculating the exergy inputs and outputs as well as the exergy destruction. .

5.3.1. Exergy efficiency calculation for each case CASE 1: Current Plant (AHU + Heat Pump + Cooling Tower); Table 15 shows the exergy destruction and exergy efficiency of each component which are indeed good indicators to evaluate how efficiently the energy is used. High exergy efficiencies mean exploiting all the available exergy content and using energy in the most rational way. Reducing the exergy losses in either a system or a process and increasing its exergy efficiency means to use energy in a more efficient way. As we can see in Table 15, the AHU is the component with the lowest efficiency followed by the heat pump and the cooling tower.

Component Exergy

destruction [kW] Exergy

Efficiency [%] Heat Pump 8.21 11.03

Cooling Tower 4.17 19.92 AHU 0.88 10.10

Cold water Pump <0.1 87.50 Condense water

Pump <0.1 88.27

Table 15 - Case 1: Exergy destruction and exergy efficiency of each component

In Figure 6 is shown the exergy destruction of all the components, but each one represented as a percentage of the total exergy destroyed in the system. With 62%, the heat pump, which represents the chiller of the current plant, is the component of the cooling chain where the largest amount of exergy is destroyed. Even if the AHU appears to destroy a small amount of exergy we have to know that most of the exergy input is lost as condense water produced by the cooling process of the moist air, this water exits the AHU and has no use. The cooling tower is the second component with the largest exergy destruction, roughly 32%, due to the huge amount of air used to cool the water coming from the heat pump (hot side), since it operates on the principle of evaporative cooling process, the efficiency depends largely on the relative humidity, so the higher the relative humidity at the inlet section the lower is the exergy efficiency, increasing consequently the mass flow rate of air.

Figure 6 - Case 1: percentage of destroyed exergy

CASE 2: AHU + ERC + Solar collector. The first configuration considered as a possible solution to achieve a high exergy efficiency in the cooling chain is represented by solar collectors producing cooling power through an ejector refrigeration cycle. Table 16 shows the exergy destruction of the main components comprising the cooling chain.

Component Exergy

destruction [kW]

Exergy Efficiency [%]

Solar collector 94.72 16.41 Ejector 10.55 31.45

Generator 4.02 75.84 Tank 1.96 89.45

Condenser 1.12 44.29 AHU 0.88 10.10

Throttling Valve 0.12 95.44 Pump ERC <0.1 87.98 Pump Tank <0.1 90.10

Pump Solar Collector <0.1 90.12 Table 16 - Case 2: Exergy destruction and exergy efficiency of

each component The auxiliary components such as water pumps, heat exchangers, throttling valve and the tank have the higher exergy efficiency values, the main components instead present lower exergy efficiencies which means that a huge amount of primary energy is needed to provide the required cooling power. The components with the largest amount of exergy destroyed are the ejector and solar collector, in Figure 7 can be seen that 83.5% of the overall exergy destroyed by the entire cooling chain is destroyed by the solar collector and 9.3% by the ejector which actually represent the two main components in the cooling process since the ejector replace the compressor used by the heat pump to generate the cooling power and the heating power from the solar collector also replace the electric energy used by the heat pump and the cooling tower. Due to the low thermal efficiency of the solar collectors to convert the solar irradiance into thermal energy, a lot of exergy is destroyed in the process, even though is important to note


that with this configuration the electric energy use is reduced only to that used by the AHU which consequently reduces the use of non-renewable energy sources. The solar collector is characterized by two types of exergy inlet streams: solar energy and the exergy flow associated with the water.


In the ejector refrigeration cycle (ERC) compression can be achieved without consuming mechanical energy directly, substituting a non-renewable energy source with a renewable one. CASE 3: AHU + ARM + Solar collector In this Case the Absorption Refrigeration Machine was selected along with the solar collectors to provide the energy necessary to drive the air-conditioning system. The cooling chain consists of three subsystems: the internal subsystem represented by the AHU, and the other two, external and intermediate subsystems, that convert the energy from solar energy to get the cooling power required. In Table 17 the exergy destruction and exergy efficiency are shown for each one of the components operating in the cooling chain.

Component Exergy

destruction [kW]


Solar Collector 92.46 14.49 ARM 13.13 22.70 Tank 0.95 93.92 AHU 0.85 10.13

Cold water pump <0.1 87.50 Tank water pump <0.1 90.01

Solar c. water pump <0.1 89.90 Table 17 - Exergy destruction and exergy efficiency of each

components The ARM is considered as a black box where there are fluxes entering and exiting the machine, the main fluxes are the hot water that enters the generator and the cold water leaving the evaporator that goes then to the cooling coil of the AHU. In this case the exergy output or useful effect is made in the evaporator and it is related directly to the refrigeration load.

As shown in Fig. 8 the solar collector is the component which destroys more exergy, followed this time by the ARM.


CASE 4: AHU + ARM + PVT hybrid solar panels. In this configuration the PVT hybrid panels that convert solar radiation into thermal and electrical energy has been introduced. We already can expect an improve in the cooling chain, from a global point of view, since there will be an output of electric power and from the panel point of view there will be also an increase of the efficiency. All the components work under the same conditions than Case 3, the only difference is the replacement of solar collectors by PVT panels. Table 18 shows the results of exergy destruction and exergy efficiency of each component.

Component Exergy

destruction [kW]


PVT Panel 111.08 17.94 ARM 13.15 22.68 Tank 0.95 93.93 AHU 0.85 10.13 Cold water pump <0.1 87.50 Tank water pump <0.1 90.01 PVT water pump <0.1 89.90 Table 18 - Exergy destruction and exergy efficiency of each

components Is worth noting that with this configuration the air-conditioning system becomes totally sustainable since the electric output can be used to feed the AHU and the auxiliary components. In this way the use of electrical energy from non-renewable primary sources is completely removed. As shown in Fig. 9 the ARM and the PVT are still the components who destroy more exergy whereas the auxiliary components do not have a big influence. CASE 5: AHU +ERC+ PVT hybrid solar panels. The last configuration is the PVT hybrid panels coupled with the ERC to provide the cooling power necessary to drive the


air-conditioning system. The ERC works as in Case 2 at the same temperatures either in the power and the refrigeration loops which means that the generation, evaporation and condense temperature are the same.


Table 19 shows the exergy destruction and exergy efficiency of each component.

Component Exergy

destruction [kW]


PVT Panel 127.61 18.74 Ejector 10.42 31.45

Generator 4.02 75.84 Tank 1.96 89.45

Condenser 1.12 44.29 AHU 0.83 10.15

Throttling Valve 0.12 95.44 ERC pump <0.1 87.98

Tank water pump <0.1 90.10 PVT water pump <0.1 90.12

Table 19 - Case 5: Exergy efficiency and exergy efficiency of each component

The components of the power loop of the ERC destroy the most exergy in the whole cooling chain, being the PVT panels and the ejector those where the most part is destroyed. The pumps and auxiliaries have a small influence in the exergy destruction. The total exergy destroyed in the process is 146.2 kW.

5.3.2. Global exergy efficiency The global exergy efficiency of each plant configuration is computed considering each whole system as a black box, the useful effect is always to cool the air so it depends on the refrigeration load. The exergy supplied is represented by all the energy inputs necessary to achieve that cooling process. When summing the exergy of the different fluxes is really important to know that the exergy quality factors have been considered, these factors multiply the exergy flow depending on the type of source we are dealing with. For example, the solar irradiation has to be multiplied by 0.95 which represent

the maximum amount available that we can exploit. In the case of electric power, the kilowatts have to be divided by the average national energy conversion factor, generally this varies from 0.4 to 0.6, for this case was assumed 0.45 (Italian energy conversion factor) because of lack of information in Venezuela. Table 20 shows the values of exergy input and output as well as the global exergy efficiency of each case.


Figure 11 presents the exergy input highlighting the part represented by solar irradiation so it can be easier to identify how much exergy comes from solar irradiation and how much from non-renewable sources.

Figure 11 - Exergy input and output of the different plant

configuration The results of the global exergy efficiencies are shown in Table 20. As we can see, in every case the global exergy efficiency is quite low due to the big amount of exergy that is destroyed in the process. During the cooling process of moist air inside the AHU a part of the water vapor in the mixture condenses as liquid water and this represents an exergy loss which is the same in each case as indicated in Table 20.


The exergy consumption of the circulation pumps and air circulation fans inside the AHU is accounted for by calculating the electric power used.

Case Exergy Input [kW]

Exergy Output [kW]

Exergy Destruction

[kW]

Exergy losses [kW]

Global Exergy

Efficiency [%]

1 45.97 0.98 37.16 7.83 2.13 2 128.86 0.98 120.05 7.83 0.76 3 123.37 0.98 114.56 7.83 0.79 4 150.65 9.64 133.18 7.83 6.40 5 163.04 9.49 145.72 7.83 5.82

Table 20 - Global exergy fluxes and efficiency of each case The cases presented as possible solutions to replace the current air-conditioning plant certainly do not have a smaller exergy input than the original plant but two of them are not less efficient either. The fact that they present a higher exergy input is due to the very low thermal-efficiency of the solar collectors and PVT panels which in turn implies that to provide for the required thermal energy, a large amount of solar energy is used. Cases 2 and 3 have an exergy efficiency of 0.76% and 0.79%, respectively, which is actually lower than the 2.13% of Case 1. Case 5 is the second possible option according to its exergy efficiency (5.82%) even though, as shown in section 4.2, the plan surface required for the PVT panels exceeds the free-surface available in the building. Finally, Case 4 is the most convenient option to substitute the current air-conditioning plant, since it has the highest exergy efficiency and the panels surface is adapted to that available in the building. It is important to emphasize that Case 4 represents a totally sustainable plant able to satisfy its own electricity demand thanks to PVT hybrid panels. 6. CONCLUSIONS An exergy analysis and optimization of the air conditioning system of the TOTAL S.A. offices located in Caracas, Venezuela has been carried out in order to achieve a reduction of the global electric energy use of the considered building. Several air-conditioning plant configurations have been simulated and compared with the actual plant currently in operation in the building, in order to select the most efficient one. These configuration plants are based on the use of solar energy as primary energy source that, coupled with an intermediate refrigeration system, provides for the necessary cooling power to cool the air into the Air Handling Unit. The results of the simulation of the current air-conditioning plant (Case 1) show that the electric energy use matches with the results presented by Ankah W. and Del Portillo H. in their study, confirming that the components modeled on Camel operate correctly according to the real devices and real working conditions.

From the simulations results was revealed that large amounts of exergy are destroyed in the main components (AHU, heat pump and cooling tower), thus resulting in a low exergy efficiency with a large amount of high-quality exergy destroyed. The alternative cases have something in common, the low efficiency of the solar collector and PVT panels and consequently they destroy most of the exergy input, in fact approximately 80% of the exergy received from solar irradiation is destroyed during the conversion process of solar energy into thermal energy or solar into thermal plus electric energy depending on the case. The low efficiency and high exergy destruction implies the use of big surfaces to provide for the thermal energy necessary to drive the refrigeration devices, even though in each of these cases the electric energy use was reduced in almost 70% for cases 2 and 3 and 100% for cases 4 and 5 due to the electric power output obtained from PVT panels. Using the electric power from PVT panels to feed the AHU, the water pump and auxiliaries makes the plant a sustainable system capable of converting solar energy into thermal and electric energy. A global analysis of each configuration plant was also made in order to evaluate their exergy efficiency, the systems were considered as black boxes with matter streams entering and exiting. It was found that cases 2 and 3 are the less efficient, so these alternatives are not viable to solve the problem, whereas cases 4 and 5 have a remarkable performance compared to the current plant. Case 5 requires a PVT panel surface bigger than that available on the building rooftop. The big panel surface required by each plant is mostly due to the low thermal efficiency of the panels. NOMENCLATURE cp Specific heat at constant pressure (kJ/kg K) Chemical molar exergy (kJ/mol) Physical molar exergy (kJ/mol) Total molar exergy (kJ/mol)

Molar enthalpy (kJ/mol) p Pressure (kPa)

Universal gas constant (kJ/mol K) Molar entropy (kJ/mol K)

T Temperature (K) Molar fraction of the i-th component

exergy efficiency

µ Chemical potential (kJ/mol) ω Humidity ratio (kg vapor/ kg dry air) Subscripts 0 Dead state a Dry air ch Chemical ma Moist air ph Physical v Water vapor


Acronyms AHU Air Handling Unit ARM Absorption Refrigeration Machine ERC Ejector Refrigeration Cycle HVAC Heating, Ventilating and Air Conditioning PVT Photovoltaic-Thermal hybrid panels 7. REFERENCES

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