Performance of a Solar Thermal Adsorption Cooling System

PERFORMANCE OF A SOLAR THERMAL ADSORPTION COOLING SYSTEM BASED ON METAL ORGANIC FRAMEWORKS IN TEXAS

Dervis Emre Demirocak Texas A&M University-Kingsville

Kingsville, TX, USA [email protected]

M. M. Kabir Texas A&M University-Kingsville

Kingsville, TX, USA [email protected]

ABSTRACT

In this study, the performance of the basic adsorption cooling system based on a metal organic framework, HKUST-1, is investigated and compared with that of a zeolite based system. The optimal regeneration temperature to maximize the COP of the HKUST-1−water based basic adsorption cycle is presented. The solar-thermal powered adsorption chiller model running on the HKUST-1−water based basic adsorption cycle is developed and integrated into a building model (two-story house located in Kingsville, Texas) in TRNSYS. The yearly performance of the integrated system is simulated by employing the latest typical meteorological year data (TMY3) for Kingsville, Texas. The solar fraction of the solar-assisted adsorption cooling system is also presented.

INTRODUCTION Texas is oil and natural gas rich, and the largest CO2 emitting state by far in the United States. Texas has also heavily invested in renewable energy technologies especially wind energy; as of 2015, around 10% of the total electricity production is provided by renewables. In addition to extensive wind potential, due to its geographical location (i.e., favorable insolation) and availability of vast open lands, solar energy also has a huge potential in Texas. However, utilization of solar energy in Texas lags far behind most of the states, and only 1% of total renewable electricity production is provided by solar photovoltaics [1]. Solar thermal technologies for power generation and building heating/cooling attracted even less attention in Texas. Buildings in Texas account for 40% and 70% of the state’s total energy and electricity use, respectively [2]. Therefore, there is significant room for energy savings in buildings. One of the ways to reduce the energy demand in buildings is to make use of solar thermal technologies for heating and cooling applications. In general, cooling demand of a building is in phase with the intensity of solar radiation (i.e., the higher the solar radiation at a specific geographical location,

the higher the cooling demand). Since the southeast Texas is hot and humid as well as sunny most of the year, solar thermal cooling is an adequate technology that can help save energy. In addition to energy saving potential, solar thermal energy can also help peak demand shaving in summer months where demand for air conditioning (A/C) is the highest. The adsorption cooling systems have become the focus of considerable attention recently because they utilize environmentally benign working fluids which do not contribute to ozone depletion or global warming, have low maintenance and operation costs, are not noisy (due to absence of rotating parts), and most importantly, can be driven by low grade waste heat or renewable energy sources such as solar or geothermal energy [3, 4]. However, the main drawbacks of the adsorption cooling systems are low coefficient of performance (COP) and low specific cooling power. To address these issues several approaches have been undertaken such as improvement of regenerative heat and mass transfer in adsorbent beds by developing advanced continuous cycles, development of novel materials with better adsorption characteristics and optimizing absorbent bed structures [5, 6]. Traditional porous materials such as zeolites, silica gel and activated carbon have been investigated for adsorption cooling extensively [7]. The most important parameters that define the performance of an adsorption cooling system are adsorbate uptake at a desirable partial pressure, isosteric heat of adsorption, and adsorption kinetics. Microporous zeolites, the most popular adsorbent for adsorption cooling, can capture water at low relative pressures with steep uptake behavior; however, their regeneration is energetically demanding (i.e., requires high temperatures) due to strong interactions between water and zeolite framework. The new class of porous materials, metal organic frameworks (MOFs), offers great promise for adsorption cooling systems due to their large surface area, tunable pore size dimensions and tailorable heat of adsorption [8-10]. The

Proceedings of the ASME 2016 10th International Conference on Energy Sustainability ES2016

June 26-30, 2016, Charlotte, North Carolina

ES2016-59130

1 Copyright © 2016 by ASME

studies on MOFs for adsorption cooling are scarce; among various MOFs that were investigated, HKUST-1 exhibited one of the highest water uptake indicating its potential for the adsorption cooling applications [8, 9]. However, to the best of our knowledge, thermodynamic analysis of the HKUST-1−water based basic adsorption cycle for a wide range of temperatures has not been conducted yet. One of our goals in this study is to investigate the variation of the COP of the HKUST-1−water based basic adsorption cycle as a function of regeneration temperature to identify the optimal operating temperature. Based on the HKUST-1−water based basic adsorption cycle model, an ideal adsorption chiller model is developed and integrated into a building model of a two-story house located in Kingsville, Texas using TRNSYS. This integrated model is used to investigate the yearly performance of the adsorption chiller. The yearly performance of the integrated system is simulated by employing the latest typical meteorological year data (TMY3) developed by the National Renewable Energy Laboratory [11]. Solar fraction of the solar-assisted cooling system is calculated, and recommendations for further improvement are presented. The schematic of the integrated model is given in Fig. 1.

Fig 1. Integrated model adopted for this study.

NOMENCLATURE ΔHads Heat of adsorption, J/kg ΔHdes Heat of desorption, J/kg a Constant in Eq. (7) ACH Air changes per hour, 1/h b Constant in Eq. (7) CPads Specific heat of adsorbent, J/g·K CPref Specific heat of refrigerant, J/g·K CPwater Specific heat of water, J/g·K COP Coefficient of performance L Latent heat of evaporation of adsorbate at the

evaporator temperature, J/g mads Mass of adsorbent, kg ṁchiller Mass flow rate in the chiller loop, kg/hr P System pressure in Eq. (7) Po Saturation pressure of the refrigerant at the adsorbent

bed temperature in Eq. (7) QΔHdes Isosteric heat of desorption, J

Qev Heat removed from the evaporator, J Qih Heat required for isosteric heating, J Qdes Heat required for desorption, J QSD Sensible heat gain of adsorbent and its adsorbate

content as a result of temperature increase from Tg1 to Tg2, J

SQtot Total cooling load of the building, J/hr Ta1 Intermediate temperature during desorption, °C Ta2 Condenser temperature, °C Tg1 Intermediate temperature during regeneration, °C Tg2 Regeneration temperature, °C Thi Temperature of water entering adsorption chiller, °C Tho Temperature of water exiting adsorption chiller, °C TMY Typical meteorological year U Overall heat transfer coefficient, W/m2·K w Amount of refrigerant adsorbed on the adsorbent,

kg/kg wmax Maximum amount of refrigerant adsorbed on the

adsorbent, kg/kg wmin Minimum amount of refrigerant adsorbed on the

adsorbent, kg/kg

BASICS OF ADSORPTION COOLING Adsorption is a surface phenomenon which can be

classified into two groups; physical adsorption (physisorption) and chemical adsorption (chemisorption). Physisorption involves weak intermolecular forces such as dispersion-repulsion (van der Waals) forces and electrostatic forces between adsorbate (usually liquid or gas) and the adsorbent (porous material) surface. On the other hand, chemisorption involves strong interaction between adsorbate and adsorbent surface [12]. Although useful conceptually, the distinction between physisorption and chemisorption may not be evident for every adsorbent-adsorbate system. The operation of the adsorption based heating and cooling systems relies on physisorption because weak intermolecular interaction between adsorbate and adsorbent is desirable to utilize low grade thermal energy (< 100°C). Adsorption is an exothermic process and the heat of adsorption of an adsorbate on an adsorbent is usually 30-100% higher than the heat of condensation at the same temperature and pressure [13]. Assuming heat of adsorption is equal to heat of desorption, minimizing heat of adsorption is advantageous to improve the coefficient of performance (COP) of the adsorption based cooling systems.

The basic adsorption cycle relies on the adsorption of a refrigerant vapor (adsorbate) on a solid adsorbent at low pressure, and the subsequent desorption at a high pressure by heating the adsorbent. In the simplest case an adsorption refrigerator can be considered as two vessels connected to each other, one of which is filled with a porous adsorbent and both of which contain refrigerant as shown in Figure 2.

At the beginning, Fig 2(a), system is at a low pressure and temperature, and the adsorbent is ideally saturated with the refrigerant (wmax) at this point. The adsorbent is heated and


refrigerant starts to desorb from the adsorbent, which increases the system pressure, Fig. 2(b). The desorbed refrigerant condenses in the second vessel and rejects heat as a result of condensation, Fig. 2(c); at this stage, adsorbent has minimum amount of refrigerant (wmin). Finally, Fig. 2(d), the adsorbent is cooled down to ambient temperature causing refrigerant to readsorb on the adsorbent producing the cooling effect [4]. The basic adsorption cycle is inherently intermittent due to the required time for the regeneration of the adsorbent bed. By using at least two adsorbent beds this basic cycle becomes a continuous basic adsorption cycle with one reactor being regenerated while refrigerant is adsorbed on the other one.

Fig. 2. Basic adsorption (intermittent) cycle, adapted from [4].

The basic adsorption refrigeration cycle consists of four

thermodynamic steps. The variation in the amount of refrigerant adsorbed on the adsorbent (w) throughout the cycle can be represented by a P-T-w diagram as shown in Fig. 3. The cycle begins at point 1 where the maximum amount of refrigerant is adsorbed. The adsorbent is at a low temperature and pressure at point 1. Along line 1-2, the adsorbent is heated and desorbs refrigerant isosterically (i.e., at constant total adsorbed mass on the adsorbent). This step is isosteric because there will be no refrigerant flow until the pressure inside the adsorbent bed becomes equal or greater than the pressure of the condenser, and the amount of desorbed refrigerant is small relative to the total amount adsorbed (i.e., Δmads/mads ≈ 0 along line 1-2). Continued heating (line 2-3) desorbs more refrigerant, forcing it to the condenser until state 3 is attained, at which desorption ceases. This second step is isobaric desorption. Then the hot adsorbent is cooled isosterically causing depressurization (line

3-4). When the pressure drops below evaporator pressure (Pev), refrigerant in the evaporator starts to boil and then it flows to the adsorbent bed, producing the cooling effect. Cooling of the adsorbent continues until the bed is saturated with the refrigerant; hence, completing the cycle. This process (line 4-1) is also isobaric adsorption.

The COP of the basic adsorption cycle can be expressed as:

desih

ev

QQQCOP+

= (1)

where Qev is the heat removed from the evaporator, Qih is the heat required for isosteric heating (line 1-2 in Fig. 3) and Qdes is the heat required for desorption (line 2-3 in Fig. 3).

Fig. 3. P-T-w diagram of the basic adsorption cooling cycle.

THERMODYNAMIC MODEL OF A REVERSIBLE ADSORPTION CHILLER

In order to calculate the COP of the basic adsorption cooling cycle, heat input to the system required for regeneration (steps 1-2 and 2-3 in Fig. 3) and the useful refrigeration effect must be calculated. Thermodynamic model of a basic adsorption chiller is given below [14, 15].

Considering Fig. 3, heat is added to the adsorbent bed in isosteric heating (1-2 in Fig. 3) and isobaric desorption (2-3 in Fig. 3) steps. The heat that must be supplied to the adsorbent bed for its isosteric heating (1-2 in Fig. 3) is given as:

))(( 21max agppadsih TTwCCmQrefads

−+= (2)

where mads is the mass of the adsorbent, CPads is the specific heat of the adsorbent, CPref is the specific heat of the refrigerant, Ta2 is the condenser temperature and Tg1 is the temperature reached at the end of the isosteric heating step which can be calculated utilizing adsorption isotherms of the adsorbent-refrigerant pair.

The heat necessary for the desorption step (2-3 in Fig. 3) has two components:

SDHdes QQQdes+= D (3)

where QΔHdes is the isosteric heat of desorption which is given by:


∫D=D

max

min

w

wdesadsH dwHmQ

des (4)

and QSD is the sensible heat gain of adsorbent and its adsorbate content as a result of temperature increase from Tg1 to Tg2:

∫+−=2

1

)()( 12

g

g

refads

T

TpadsggpadsSD dTTwCmTTCmQ (5)

Finally, heat removed from the evaporator is given as: )()( minmax evadsev TLwwmQ −= (6)

where L(Tev) is the latent heat of evaporation of adsorbate at the evaporator temperature.

Using equations 1-6, one can calculate the COP of a basic adsorption chiller for a specific adsorbent-refrigerant pair. Following assumptions are instrumental in calculating the COP: (i) specific heat of the adsorbent and refrigerant are assumed to be constant, (ii) specific heat of the refrigerant in the adsorbed phase is assumed to be equal to the specific heat of the bulk liquid phase [14, 16], (iii) heat of adsorption is assumed to be equal to the heat of desorption and (iv) thermal contributions of refrigerant vapor, metal parts and additives are neglected.

The specific heats of different adsorbents are seldom reported in the literature, and this is the main reason behind the constant specific heat assumption for the adsorbents in this study. As will be discussed in the next section, COP of the adsorption cooling system is calculated for the temperature range of 55-120°C. In this range, assuming constant specific heat for water (refrigerant) introduces an error around 1.5% which is negligible. On the other hand, assuming constant specific heat for the adsorbent introduces an error up to 10% for HKUST-1 and Zeolite 4A [17]. However, the error introduced by constant specific heat assumption for the adsorbent affects both HKUST-1 and Zeolite 4A in a similar way (i.e., Cp of both adsorbents increases by temperature); therefore, it does not introduce significant error when comparing the performance of HKUST-1 with Zeolite 4A as given in Fig. 4.

COMPARISON OF COEFFICIENT OF PERFORMANCE OF HKUST-1−WATER AND ZEOLITE 4A−WATER

One of our goals in this study is to compare the performance of an adsorption chiller based on novel microporous materials such as MOFs (i.e., HKUST-1) with the conventional microporous materials such as zeolites (i.e., Zeolite 4A). Previously we studied the performance of the Zeolite 4A−water pair [18]. Water is preferable as a refrigerant because it is environmentally friendly, low cost and ubiquitously available, and has a high enthalpy of evaporation as compared to other common refrigerants utilized in adsorption cooling applications such as methanol and ammonia.

The accurate calculation of the COP of a basic adsorption chiller at different operating temperatures depends on the availability of the accurate adsorption isotherm data of the adsorbent-refrigerant pair at various temperatures which is only

reported for a limited number of MOFs in the literature. Despite the moderate stability of HKUST-1 when subjected to water [9], adsorption isotherms of HKUST-1-water pair at various temperatures are given in the literature [8]. Based on the experimental adsorption isotherms of HKUST-1-water, the following Langmuir model based correlation with an accuracy of ±15.5% is utilized for the COP calculations [8, 19]:

)(1o

o

PPb

PPb

aw+

= (7)

where a and b are constants and equal to 0.6576 and 4.548, respectively. P is the system pressure and Po is the saturation pressure of the refrigerant at the adsorbent bed temperature. The uncertainty associated with the Eq. (7) will introduce similar level of error (i.e., ±15.5%) in COP calculations for HKUST-1.

Based on the adsorption isotherms simulated by using equation 7, wmin and wmax values are calculated for different regeneration temperatures. Specific heat of HKUST-1 and water are 0.8 and 4.187 J/g·K, respectively [17], isosteric heat of adsorption for HKUST-1−water pair is 2693 J/g [8], and the heat of evaporation of water, L(T), is calculated using the following relation [20]:

TTL 4425.23172)( −= (8) where T is in K and L(T) is in J/g.

Fig. 4. The COP of HKUST-1−water and Zeolite 4A−water as a function of regeneration temperature.

The COP of the HKUST-1−water and Zeolite 4A−water based basic adsorption cycle as a function of regeneration temperature is given in Fig. 4. The HKUST-1−water shows significantly better performance at low regeneration temperatures. The COP of HKUST-1−water pair peaks around 60°C (~0.72) and then gradually decreases (~0.65 at 120°C).


On the other hand, the COP of Zeolite 4A−water pair steadily increases from 0.35 at 55°C to 0.5 at 120°C.

Results clearly indicate that HKUST-1−water is preferable over Zeolite 4A−water at low regeneration temperatures (< 120°C). On the other hand, there are other types of zeolites that perform on par with HKUST-1 such as AQSOA-Z01 developed by Mitsubishi Plastics [9, 21].

BUILDING ENERGY SIMULATION As mentioned in the Introduction, one of the goals of this

study is to simulate the performance of an adsorption chiller for a building located in Kingsville, Texas. The building model is created using TRNSYS 3D extension in SketchUp™ software as shown in Fig. 5(a).

Fig. 5. (a) TRNSYS 3D-SketchUp™ model and (b) floor plan of the building.

The accuracy of the weather data is critical in building energy simulation. Recently, National Renewable Energy Laboratory (NREL) released the updated typical meteorological

year (i.e., TMY3) data for various US locations [11]. The TMY3 data of Kingsville, Texas is used to simulate the yearlong energy performance of the building.

The floor plan of the building is given (attic space is not shown) in Fig. 5(b), and the building is divided into five different zones as follows: (i) Zone 1 → Living room + kitchen + stairwell + corridor, (ii) Zone 2 → 1st room, (iii) Zone 3 → 2nd room, (iv) Zone 4 → Bathroom and (v) Zone 5 → Attic (not air conditioned).

Building cooling and heating loads are lower in northern hemisphere when the more extensively glazed facade is oriented to the south compared to the same building oriented to the east, west or north [22]. Since Kingsville, Texas is located in northern hemisphere (27.5150° N, 97.8656° W), the facade of the building with the largest window area is oriented towards south as shown in Fig. 5(a) and Table 1. The overall U-values for different building elements are given in Table 2. Windows have a U-value of 2.89 W/m2·K and g value of 0.789.

Table 1. Window areas on the building’s facades.

Zones Window areas (m2) North South East West

1 2 6 3 4 2 − 2 − − 3 − 2 − − 4 − − − −

Table 2. U-values of the building components.

Building element

External wall

Internal wall

Ground floor

Ceiling wall1

Roof wall

U-value (W/m2·K) 0.510 0.508 0.039 4.153 0.316

1 Ceiling wall between the 2nd floor and the attic space (i.e., zone 5) is considered as a roof wall.

Internal gains included in the model are as follows: (i) It is assumed that building is always occupied by 2 people doing light work (150 W), one in zone 1 and the other one in zone 2, (ii) Artificial lighting of 5 W/m2 that is on at all times is included in zones 1 and 2 and (iii) To account for the appliances, 230W source that is on at all times is included in zones 1 and 2.

Infiltration is taken as 2.0 air changes per hour (ACH) for all zones and ventilation is taken as 0.5 ACH for zones 1-4. To estimate the building cooling load, temperature and relative humidity set points are taken as 22°C (71.6°F) and 50%, respectively. In calculations, it is assumed that no shading device is utilized and there is no external shading.

Our preliminary analysis indicated that, in Kingsville, Texas, active cooling is mostly necessary from the beginning of May to the end of November. From the beginning of December

(a)

(b)


to the end of April either heating is necessary or there is no need for heating or cooling. In case cooling is needed from the beginning of December to the end of April, this can be simply provided by supplying ambient air to the building. The variation of the ambient and zone 1 temperatures as well as the total and latent cooling load of the building is given in Fig. 6. The fluctuation of the zone 1 temperature shown in Fig. 6(a) indicates that, heating is needed more on average compared to cooling during May and November; especially at night times considering the set point temperature for heating is 20°C (68°F). The heating demand and the feasibility of providing heating using an adsorption heat pump are not discussed in this study, but will be addressed in future works. For brevity, only total and latent cooling load of all air conditioned zones (1-4) combined are given in Fig. 6(b). The maximum cooling load of 8.71 kW occurs on August 19. The significant portion (~75%) of the cooling load is originating from zone 1.

Fig. 6. (a) Variation of the ambient and zone 1 temperatures and (b) the total and latent cooling load of the building based on

the temperature and relative humidity set points of 22°C (71.6°F) and 50%, respectively.

INTEGRATED TRNSYS MODEL FOR THE SOLAR-ASSISTED ADSORPTION COOLING SYSTEM

There are two approaches in designing solar air-conditioning systems in buildings: (i) solar-thermally autonomous systems and (ii) solar-assisted systems. The solar-thermally autonomous systems are self-sufficient systems. This type of systems may not always satisfy the desired indoor conditions, and statistical analysis is used to evaluate how often indoor temperature and humidity will exceed certain comfort requirements. Also cooling loads and solar gains should be very well synchronized in these systems. Usually solar-thermally autonomous systems are reasonable when using a back-up system is not feasible or recommended. On the other hand, the solar-assisted systems are used to reduce conventional energy usage by using solar energy. These systems use back-up systems to provide the required amount of cooling under all circumstances. The back-up system may be a second heat source like a gas-fired boiler or a conventional vapor compressed chiller for generating cooling power directly. In this study, solar-assisted cooling system with a back-up heat source (i.e., auxiliary heater embedded in the storage tank) system is emphasized. This configuration is the most widely used system among the solar heat driven chiller based systems [23]. The schematic of the solar-assisted adsorption cooling system is given in Fig. 7.

Fig. 7. The schematic of the solar-assisted adsorption cooling system.

Mainly, there are two loops in the solar-assisted adsorption

cooling system as shown in Fig. 7, collector and chiller loops. The third loop (not shown) between the adsorption chiller and the building cooling load is not considered in this study because adsorption chiller model is based on the ideal thermodynamic cycle presented in the preceding sections (i.e., no heat or mass transfer limitations). The temperature of the water exiting the adsorption chiller (Tho) is calculated using the following equation:

COPSQTTCm tot

hohipchiller water=− )( (9)

where ṁchiller is the mass flow rate of the chiller loop given in Fig. 7, Cpwater is the specific heat of water, Thi and Tho are the temperatures of hot water entering and exiting the adsorption


chiller, respectively, and SQtot is the total cooling load of the building. The COP values from 55-120°C for the HKUST-1−water pair are calculated as given in Fig. 4, and the total building cooling is calculated via TRNSYS as presented in Fig. 6.

The integrated TRNSYS model for the solar-assisted adsorption cooling system is given in Fig. 9. As shown in Fig. 4, the optimal regeneration temperature for the HKUST-1−water based adsorption chiller is around 60°C and this is the most prominent advantage of this adsorbent-refrigerant pair as compared to Zeolite 4A−water. Besides the thermodynamic analysis, lower regeneration temperature is also desirable from the economic standpoint since it lowers the operating (i.e., auxiliary heating demand will be less) and capital costs (lower cost flat plate collectors can be utilized as compared to evacuated tube collectors).

The operating parameters of the integrated system are selected to maximize the COP of the adsorption chiller. The following assumptions are made to simulate the performance of the adsorption chiller via TRNSYS: (i) Slope of the flat plate collectors is equal to the latitude of Kingsville, Texas (i.e., 27.5°), (ii) Collectors are directly facing the equator (azimuth angle, α=180° in reference to true north), (iii) There is no tracking device (i.e., collectors are fixed), (iv) Total collector area is 20 m2, (v) Mass flow rates of the collector and chiller loops are 1080 and 540 kg/h, respectively, (vi) Storage tank volume is 1 m3, (vii) Set point temperature for the auxiliary heater embedded to the storage tank is 55°C to operate the HKUST-1−water based basic adsorption chiller near optimal regeneration temperature of 60°C.

Fig. 8. The integrated TRNSYS model for the solar-assisted adsorption cooling system.

The controller in the collector loop shown in Fig. 8 is in place to minimize the heat losses from the collector field; the controller stops the pump when the collector outlet temperature is lower than the collector inlet temperature. Basically, this controller stops the collector loop flow when there is no sunshine (i.e., during night time).

Table 3. Solar fraction of the HKUST-1−water based solar-assisted adsorption cooling system for each month.

Months May Jun Jul Aug Sep Oct Nov

Solar fraction 0.95 0.75 0.60 0.61 0.70 0.76 0.98

In the design of solar-thermal systems, solar fraction is one

of the important parameters that needs to be considered. Solar fraction is the ratio of the energy provided by the solar technology divided by the total energy requirement of the system. It is recommended that solar-thermal systems should have a solar fraction of 0.7-0.8 or higher to save significant amount of primary energy [24].

Fig. 9. Variation of the total (i.e., solar and auxiliary heater) and auxiliary heater energy rate provided to the adsorption chiller and the solar fraction of the system during the 1st week of August.

The solar fraction of the HKUST-1−water based solar-

assisted adsorption cooling system for each month is given in Table 3. The solar fractions in May and November are considerably high (< 0.95) because during these months cooling is only required during day time when there is abundant insolation available and the required heat input for the adsorption chiller can easily be provided by the flat plate collectors. The solar fractions for months July and August are around 0.6. The solar fractions in these months are lower compared to other months because during these months cooling


demand is very high, see Fig. 6(b), and cooling is required almost 24 hours a day. To better explain this point, refer to Fig. 9 where the total (i.e., solar and auxiliary heater) and the auxiliary heater energy rate provided to the adsorption chiller as well as the corresponding solar fraction of the system during the first week of August are provided. As seen from Fig. 9, solar fraction switches between 0 and 1 in a typical day; during day time when there is abundant insolation, solar fraction is 1; during night time when there is no insolation, solar fraction is 0 (i.e., all the heat input is provided by the auxiliary heater). To improve the solar fraction in July and August, storage tank with a phase change material can be utilized.

CONCLUSIONS In this study, thermodynamic analysis of the basic

adsorption cooling cycle is presented. The basic adsorption cooling cycle based on MOFs (i.e., HKUST-1) showed significantly better performance than the Zeolite 4A. The optimal regeneration temperature to maximize the COP of the HKUST-1−water based basic adsorption cooling cycle is around 60°C which is in stark contrast with the Zeolite 4A−water pair. Since HKUST-1−water pair performs better at lower regeneration temperatures there is no need to employ evacuated tube solar collectors which is desirable to lower the capital costs. Additionally, yearly cooling load of a two-story house located in Kingsville, Texas is calculated via TRNSYS. Results revealed that, the cooling demand is the highest in July and August. The performance of the ideal HKUST-1−water based basic adsorption cooling cycle is investigated using an integrated model built in TRNSYS. It is shown that, it is possible to obtain a solar fraction of 0.6 or higher from the beginning of May till the end of November by utilizing the proposed solar-assisted adsorption cooling system.

In future work, thermodynamic analysis of the basic adsorption cycle will be expanded to include the improved adsorption cycles such as heat recovery and heat & mass recovery adsorption cooling cycles. Moreover, the performance of different working pairs will be presented. Additionally, the integrated model presented in this study will be optimized to obtain the highest solar fraction by adjusting the mass flow rates of collector and chiller loops, storage tank volume, and the solar collector area, orientation and tilt. Furthermore, the performance of the HKUST-1−water based basic adsorption cooling cycle will be investigated for arid climates (i.e., West Texas). Moreover, the feasibility of utilizing a phase change thermal storage tank will be examined to maximize the solar fraction.

ACKNOWLEDGMENTS The authors greatly acknowledge the financial support from

the Texas A&M University-Kingsville.

REFERENCES 1. American Council on Renewable Energy, Renewable

Energy in Texas, [cited Dec 12, 2015; Available from: http://acore.org/files/pdfs/states/Texas.pdf.

2. State Energy Conservation Office of Texas, Texas Building Energy Code, [cited Dec 16, 2015; Available from: http://seco.cpa.state.tx.us/tbec/.

3. Wang, D., Li, Y., Li, D., Xia, Y., and Zhang, J., A review on adsorption refrigeration technology and adsorption deterioration in physical adsorption systems. Renewable and Sustainable Energy Reviews, 2010, 14: p. 344-353.

4. Critoph, R. and Zhong, Y., Review of trends in solid sorption refrigeration and heat pumping technology. Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering, 2005, 219: p. 285-300.

5. Wang, R., Performance improvement of adsorption cooling by heat and mass recovery operation. International Journal of Refrigeration, 2001, 24: p. 602-611.

6. Akahira, A., Alam, K., Hamamoto, Y., Akisawa, A., and Kashiwagi, T., Mass recovery adsorption refrigeration cycle—improving cooling capacity. International Journal of Refrigeration, 2004, 27: p. 225-234.

7. Wang, L., Wang, R., and Oliveira, R., A review on adsorption working pairs for refrigeration. Renewable and Sustainable Energy Reviews, 2009, 13: p. 518-534.

8. Rezk, A., Al-Dadah, R., Mahmoud, S., and Elsayed, A., Characterisation of metal organic frameworks for adsorption cooling. International Journal of Heat and Mass Transfer, 2012, 55: p. 7366-7374.

9. de Lange, M.F., Verouden, K.J., Vlugt, T.J., Gascon, J., and Kapteijn, F., Adsorption-Driven Heat Pumps: The Potential of Metal–Organic Frameworks. Chemical Reviews, 2015, 115: p. 12205-12250.

10. Furukawa, H., Gándara, F., Zhang, Y.-B., Jiang, J., Queen, W.L., Hudson, M.R., and Yaghi, O.M., Water adsorption in porous metal–organic frameworks and related materials. Journal of the American Chemical Society, 2014, 136: p. 4369-4381.

11. National Renewable Energy Laboratory, 1991- 2005 Update: Typical Meteorological Year 3, [cited on Dec 12, 2015]; Available from: http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/.

12. Ruthven, D.M., Principles of adsorption and adsorption processes, 1984: John Wiley & Sons.

13. Suzuki, M., Adsorption engineering, 1990, Kodansha, Elsevier.


http://acore.org/files/pdfs/states/Texas.pdf

http://seco.cpa.state.tx.us/tbec/

http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/

http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/

14. Critoph, R., Performance limitations of adsorption cycles for solar cooling. Solar Energy, 1988, 41: p. 21-31.

15. Cacciola, G. and Restuccia, G., Reversible adsorption heat pump: a thermodynamic model. International Journal of Refrigeration, 1995, 18: p. 100-106.

16. Pons, M. and Grenier, P., A phenomenological adsorption equilibrium law extracted from experimental and theoretical considerations applied to the activated carbon+ methanol pair. Carbon, 1986, 24: p. 615-625.

17. Mu, B. and Walton, K.S., Thermal analysis and heat capacity study of metal–organic frameworks. The Journal of Physical Chemistry C, 2011, 115: p. 22748-22754.

18. Demirocak, D.E., Thermodynamic and Economic Analysis of a Solar Thermal Powered Adsorption Cooling System, Master's thesis, 2008, Middle East Technical University.

19. Hamamoto, Y., Alam, K., Saha, B., Koyama, S., Akisawa, A., and Kashiwagi, T., Study on adsorption refrigeration cycle utilizing activated carbon fibers. Part 2. Cycle performance evaluation. International Journal of Refrigeration, 2006, 29: p. 315-327.

20. Cacciola, G., Restuccia, G., and Van Benthem, G., Influence of the adsorber heat exchanger design on the performance of the heat pump system. Applied Thermal Engineering, 1999, 19: p. 255-269.

21. Goldsworthy, M., Measurements of water vapour sorption isotherms for RD silica gel, AQSOA-Z01, AQSOA-Z02, AQSOA-Z05 and CECA zeolite 3A. Microporous and Mesoporous Materials, 2014, 196: p. 59-67.

22. Andersson, B., Place, W., Kammerud, R., and Scofield, M.P., The impact of building orientation on residential heating and cooling. Energy and Buildings, 1985, 8: p. 205-224.

23. Henning, H.-M., Solar-assisted air conditioning in buildings: a handbook for planners, 2007, Springer Verlag Wien.

24. German Solar Energy Society, Planning and Installing Solar Thermal Systems: A Guide for Installers, Architects and Engineers, 2010, Earthscan.


Documents

Performance of a Solar Thermal Adsorption Cooling System