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In this paper, an Adsorption Heat Storage System (AdHS-R134a)/heating system utilising Vermiculite and Calcium Chloride composite adsorbent material was experimentally investigated. The main aim of the experimental investigations is to carry out preliminary tests on a small scale Adsorption Heat Storage Systems (AdHS-R134a) using a heat pump circuit as the regeneration heat source. The test rig was constructed using Vertical Glass Pipes with a heat pump circuit using a mini compressor for transporting the refrigeration gas as a heat source for desorption cycle. The system also incorporates condenser coils, evaporator coils, and an expansion valve. The integration with a heat pump circuit is to analyse the performance of an AdHS-R134a using off-peak power in desorption/charging cycle or utilising renewable energy sources to minimise conventional energy generated from fossil fuels. Firstly, desorption phase occurs during night hours, when cheap off-peak electricity is available under the 'Economy 7' tariff that is more suitable for households with night storage heaters or if we use lots of electricity at night. Secondly, in the heat pumping phase/adsorption loop which will occur during the day. The useful heat of adsorption in the adsorbent pipe could be used for underfloor heating (35°C-40°C), or for domestic hot water production (55°C-60°C) during the day. The maximum temperature lift observed from the adsorption process is 68.67°C (inside adsorption pipe) with the corresponding COP of 0.55-1.39.
Keywords: Thermochemical, adsorption, heat storage, R134a
In Europe, the building sector constitutes to about one-third of the total energy consumption of the Union [1,2]. The heat production from households (heating and domestic hot water (DHW)) accounts 19% [1]-24% [2] of the overall energy consumption of the UE-27. Approximately 20% is produced from renewable energy sources like wood, wastes or hydropower. Therefore, this means that approximately 80% of the heat generated in residential buildings comes from fossils resources. As cooling and heating demand for climate control rises in the future, it will lead to increasing energy consumption. This instinctively leads to a faster depletion of known fossil fuel reserves, more carbon dioxide emissions and a higher peak electricity demand. Such environmental issues have intensified research efforts on the development of environmentally benign refrigerants and energy saving cooling and heating technologies and renewed interest in heating applications. Consequently, adsorption heat storage systems considered more environmentally friendly with the ability to use water as the refrigerant for the heating and cooling in building.
Furthermore, adsorption heat storage/Thermochemical Heat Storage is a promising technology to solve the mismatch between seasonal heat supply and demand as a typical problem in temperate climate zones. Such systems can make use of solar irradiation during the summer time to cover the heat demand during the winter time. Adsorption heating/cooling was extensively investigated to compete with the conventional vapour compression systems [3-8]. The key of possessing high energy density and high sorption capacity is the crucial to the improved coefficient of performance (COP) of the adsorption systems against its counterparts. One way to solve these problems is by using composite materials in the adsorbent bed. The composite material is known as "composite salt inside matrix" have recently been recognized as the promising materials for adsorption systems due to their enhanced sorption capacity to common working fluids such as water, methanol and ammonia [9-14]. The high energy density with low regeneration temperature of composite adsorbents makes adsorption systems ideal alternative to conventional heating and cooling systems [15-17]. Thus, this paper aims to investigate the performance of an Adsorption Heat Storage System (ADHS) incorporated composite adsorbent material (CaCl2/vermiculite), and water as the working media. Other than that,desorption process is achieved by utilising a heat-pump circuit and the process conducted during night time (off-peak hour) for the cheaper energy of 'Economy 7' [18,19]. The refrigerant gas used in the heat-pump circuit is 1, 1, 1, 2-tetrafluoroethane (R134a). The integration with a heat pump circuit is to analyse the performance of an AdHS-R134a using an off-peak power in desorption/charging cycle to minimise energy demands at night time.
Principles of adsorption heat storage systems (ADHS)
ADHS/Thermochemical energy storage (TES) based on performing reversible chemical reaction [20]. Thermochemical ES processes (Figure 1) divided into three phases which consist of; i) Charging, which normally known as anendothermic reaction. The heat source is required for the dissociation process of C, ii) Storing, this stage occurs after the charging process and A and B will be formed and stored, iii) discharging, where A and B associated with an exothermic reaction and material C are regenerated and the recovered energy released [21,22].
The experimental investigation was carried out using a smallscale laboratory model located at the Sustainable Research Building Laboratory of the Department of Architecture and Built Environment, University of Nottingham, UK. The prototype system illustrated in Figure 2. The AdHS-R134a system consists of two main separate component systems, namely the heat storage system, and the regeneration system. The heat storage systems consist of the adsorption pipe, evaporator pipe and the vacuum pump. The regeneration system employs a common heat pump circuit which includes a mini-compressor, an expansion valve, a receiver, coil pipe circulating the adsorption pipe and evaporator pipe. The adsorption/evaporator pipesare two separate vertical glass vessel QVF DN50 pipes, connected to a valve, and equipped with thermocouples probes and pressure transmitter for the measurement of temperature and pressure, respectively.
Figure 2: Schematic of AdHS-HFC134a system.
The detail descriptions of main system components given in Table 1. Forthis particular rig, the K-type thermocouples were chosen. Type K thermocouple has a temperature range between -50°C and 250°C with a tolerance of ±1.5°C or ±0.4% of reading. It is particularly suitable for high pressure, high vacuum or high vibration application. The pressure and temperature measurement points illustrated in Figure 2. The pressure measured with UNIK 5000 GE pressure transmitter, which is a multipurpose, high-performance stainless steel 7-28V output transducer transmitting at 4-20mA output range. The pressure range is -1 to 1.6 bar. It is a temperature compensated strain gauge technology with a +/- 0.04% accuracy full scale. The test rig also equipped with data logging equipment DT500 for recording and monitoring the experimental data. The leakage tests conducted before the experiments begun using a vacuum pump and the pressure kept to -1bar during the test. The adsorption pipe and evaporator pipe were fully insulated to minimise heat loss during the experiments (see Figure 3).
Table 1 : Details Description of main components.
Figure 3: Enthalpy Flow of the THSS-R134a.
Firstly, the experiment started with the adsorption process by un-evacuated the Swagelok ball valve that separating the adsorption and evaporation pipes. During the adsorption process, the water vapour from the evaporator pipe below will flow to the condenser pipe due to pressure different in the systems. The adsorption process monitored until there is no temperature different in the adsorption pipe. Secondly, desorption process will be conducted. During desorption process, the adsorption pipe receives heat from the coil pipe fill with refrigerant gas from the compressor and then transfer heat to the adsorbent material. During this process, the condensate water returns to the evaporator by opening the Swagelok ball valve. The temperature and pressure were measured every one-second interval. The mass of adsorbent material and water were weight before and after adsorption and desorption processes. Two (2) test conducted with a different ratio mass of adsorbent material and water which was 1:2&1:1. These experiments aim to analyse the effect of the mass of working pair to the temperature variation for the adsorption and desorption processes. These experiments focused on the temperature lift in the condenser pipe and temperature drop in the evaporator pipe during the adsorption & desorption process. The enthalpy flow of 134a circuits for the desorption process is according to Figure 3. The analysis of enthalpy based on the P-H diagram illustrated in Figure 4. The final constructed of AdHS-r134a shown in Figure 5.
Figure 4: P-H diagram.
Figure 5: The constructed AdHS-HFC134a systems and components.
Analysis
The equations used for analysing the experimental results given in Table 2.
Table 2 : Equations used in the analysis.
Temperature variation during adsorption
The experimental data for adsorption and desorption/regeneration tests using Vermiculite - CaCl2 adsorbent, conducted using the glass pipes rig assembly was analysed, and the results discussed below.
Adsorption test
Adsorption test was initially carried out using the Vermiculite -CaCl2 adsorbent (mass ratio of 1:1) and the results of temperature variations at the adsorption andevaporation vessels of the rig shown in Figure 6. The temperature in the adsorption pipe increase gradually from an initial temperature of 24.6°C to the highest temperature of 54.56°C. The temperature was noticed to be drop sharply from 54.56°C to 45°C in 8 minutes and gradually drop to its initial temperature (22.08°C) in the 75th minute of the whole adsorption process. The temperature gain gives the maximum heating of 1727J/s and maximum cooling of 2012 J/s. The initial mass of adsorbent material of 63g increased to 67g which gives the sorption capacity of 0.06 kg dry adsorbent/kg water. The sorption capacity was considered very low, and some further improvement should be entailed to achieve this. Apart from low sorption capacity, this test evidences that the composite adsorbent material could increase the temperature through adsorption process to a suitable temperature appropriate for heating applications. Contrariwise, a sharp rise in the adsorption pipe of 68.08°C from its initial temperature of 24.92°C for Test 2 during the adsorption process (see Figure 7). It is evidence that the mass ratio of 1:2 could achieve the adsorption process relatively 20 minutes quicker than the mass ratio of 1:1. Therefore, this could be a future reference to optimize the quantities of adsorbent material and water for a larger scale design. Apart from that, the initial mass of the adsorbent material of 44g increase to 54g after the adsorption process. This gives a sorption capacity of 0.18 and relatively 0.66% higher than Test1. Furthermore, the heat release and energy drop is higher than in test 1, which is 1750J and 2280J respectively. Thus, the developed prototype is practicable and can discharge a large amount of heat (68°C) for hot water or heating demand and high sorption capacity with an appropriate ratio of adsorbent material and water mass.
Figure 6: Test 1, Temperature variation and Heat release from adsorbent material (Qheating) and Energy drop (Qcooling) from the water vapor.
Figure 7: Test 2, Temperature variation and Heat release from adsorbent material (Qheating) and Energy drop (Qcooling) from the water vapor.
Desorption test
It is necessary for the AdHS prototype system to be regenerate at a relatively low desorption temperature achieved by using low-grade energy sources such as waste heat or ground heat source. As per discussed above the regeneration was carried out using a heat pump circuit utilising R134a as the refrigerant gas. The volumetric flow rate of refrigerant gas was constant at 1.9 cm3/rev through the 4 hours of regeneration. Figure 8 illustrates the temperature variation from desorption process for Test 1. It can be seen that the HP (high pressure) inlet temperature increased gradually from an initial temperature of 24°C to 62.13°C in 100 minutes. Then the (HPinlet temp) remain constantly about 63°C through 4 hours of regeneration. The temperature in the adsorption pipe (AdsTemp) increased from 25°C to 49°C in the first 66 minutes. Then, the (Adstemp) remain constant at 54°C in 4 hours of the regeneration process. This result showed that there were 9°C (ΔT)temperature different from the coils pipe to the (Adstemp). Therefore, these experiments suggest that that there are some heat losses from the copper pipe transferring the heat to the glass vessel which is technically low thermal conductivity of 1.2 W/m2K. Then, the liquid refrigerant was being transferred through copper coils from the adsorbent pipe to the evaporator pipe. The temperature variant to the evaporator pipe indicated as (evaptemp) and the refrigerant gas flows through copper coils is indicate as LPinlet/outlet temp. It can be seen that in the (LPinlet) and (evapTemp) has the temperature different about 7°C which indicates that temperature loss also occurs in the evaporation sections. However, lower temperature of theaverage of 1.5°C in the evaporator side is sufficient enough condensate the water vapour from the desorption process. In a real scale of AdHs systems, the ground heat source shall be utilized as a heat source for the heat pump and improve the performance of the whole systems. Similarly in desorption Test 2, (see Figure 9), temperature variant in the adsorption section and evaporator section experiencing a temperature loss of approximately about 10°C. However, the temperature in the evaporator pipe is higher that Test 1. This may be due to the amount of water used more than in Test1 where longer time was needed to condense the water vapour. Additionally, the mass of adsorbent and water were re-weight after desorption process, and the same amount of water being desorb after this process. Therefore, these experiments suggest that the regeneration temperature for Vermiculite+CaCl2 is in the range of 55°C to 65°C. Thus, a low-grade regeneration system such as flat plate collector or ground source heat pump could be utilized to reduce demand for using a conventional energy source from a fossil fuel.
Figure 8: Desorption test for test 1(mass ratio: 1:1), HPinlet/outlet (gas R134a from compressor and to the expansion valve), LPinlet/outlet (liquid from HFC134a from expansion valve to compressor).
Figure 9: Desorption test for test 2 (mass ratio: 1:2), HPinlet/outlet (gas R134a from compressor and to the expansion valve), LPinlet/outlet (liquid from R134a from expansion valve to compressor.
Overall system performance
Table 3, showing the overall performance of the AdHS-HFC134a of the different mass ratio which is 1:1 and 1:2 respectively. The heat store (Qheating store) of Test 2 is higher than TEST 1 by approximately 30% more. It can be seen that the heat storage strongly influenced by the increase of mass of adsorbent material. Furthermore, the time (s) for temperature lift to maximum values (ΔTmax) for Test 2 was approximately 70% quicker than that of Test 1. This would affect adversely to the COP of the systems which is the maximum COP for Test 2 is 1.39 compared to 1.16 for Test 1 respectively. Therefore, the overall performance of the system is highly dependent on the mass of the working pair and the temperature lift during the adsorption process. Thus, these experiments suggest that an actual scale system should use a most appropriate mass for working pair to enhance the heat store and COP the whole systems. Besides, the heat source should be constructed inside the reaction vessel (adsorption vessel) to enhance the heat transfer and improve the dissociation of water vapour during the desorption process.
Table 3 : Performance comparison between Test 1 and Test 2.
This paper represents the experimental investigation of a prototype scale of an Adsorption heat storage system using heat – pump circuit (R134a) as a heat source for desorption. The following remarks can be drawn from the present study:
The authors declare that they have no competing interests.
| Authors' contributions | NMW | BM | AD | SO | SF |
| Research concept and design | √ | √ | √ | √ | √ |
| Collection and/or assembly of data | √ | √ | √ | -- | -- |
| Data analysis and interpretation | √ | √ | √ | -- | -- |
| Writing the article | √ | √ | √ | -- | -- |
| Critical revision of the article | √ | √ | √ | √ | √ |
| Final approval of article | -- | -- | -- | √ | √ |
| Statistical analysis | √ | √ | √ | -- | -- |
The first author wish to express her acknowledgements to Public Works Departments (PWD) and Public Services Departments of Malaysia (PSD) for their scholarship support associated with this study. The work of this paper was supported by the Department of Energy & Climate Change (DECC) (RGS: 109613).
EIC: Saffa Riffat, University of Nottingham, UK.
Received: 05-Mar-2016 Final Revised: 15-Apr-2016
Accepted: 25-Apr-2016 Published: 04-May-2016
Copyright © 2015 Herbert Publications Limited. All rights reserved.
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