Study of Adsorbent Energy Density and Regeneration for Long Term Thermal Energy Storage Of Solar and Waste Heat

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1 Study of Adsorbent Energy Density and Regeneration for Long Term Thermal Energy Storage Of Solar and Waste Heat Dan Dicaire and F. Handan Tezel (*) Department of Chemical and Biological Engineering University of Ottawa, 161 Louis-Pasteur, Ottawa, Ontario K1N 6N5 CANADA Abstract A lab scale system has been developed in this study to demonstrate long term thermal energy storage for space heating using adsorbent beds. The reversible exothermic adsorption process releases heat when exposed to humidity and can be regenerated using low grade heat obtained from solar or waste heat. The energy does not degrade with time and the system can be recharged repeatedly without significant loss of performance. The level of regeneration/energy storage achieved from various regeneration conditions has been studied. The practical feasibility of the system is verified by applying the resulting regeneration parameters, efficiency, and energy densities to typical residential space heating needs. The experiments were performed on a new hybrid material as well as a natural zeolite in order to confirm that the trends are reproducible in different adsorbents. 3 different scenarios have been considered for this seasonal thermal storage and corresponding adsorbent beds have been sized. Introduction Solar energy is the most abundant and accessible source of energy on earth. However, it is not constant through the seasons producing an excess during the summer and a deficiency during the winter. The development of more environmentally responsible energy practices is pushing to increase efficiencies and find value in previously unusable waste. The key to utilizing wasted solar heat is storage; to accumulate energy when it is in excess and use it when it is lacking. Although there are many different types of Thermal Energy Storage (TES) systems, the focus of this paper is on Long Term Adsorption Thermal Energy Storage and its many advantages over typical sensible thermal storage system like aquifers or water tanks. Adsorption is an exothermic reversible process. When a bed of adsorbent is exposed to humid air, adsorption of water takes place and the exothermic process releases heat. Once the adsorbent is saturated, the system can be regenerated by introducing heat (provided by solar energy or waste heat) to release the adsorbed water and regenerate the adsorbent so that it is ready to adsorb water again. The stored energy does not degrade with time as long as the adsorbent is kept dry. They can also be recharged repeatedly with limited loss of performance [9]. (*) Author to whom correspondence should be sent to: handan.tezel@uottawa.ca 1

2 Long Term Adsorption TES have been developed in the past for a variety of applications. The technology has been proven in large scale systems using electric resistance during off peak times to regenerate the adsorbent. Systems typically report an energy density between kwh/m 3 [3] but theoretically could reach up to kWh/m 3. A prototype system has been developed to study the process, optimize the energy density, and characterize the regeneration parameters. The application considered in this study is space heating since 60% of residential energy use and 52% of commercial energy use in Canada goes towards space heating [6]. The heat for the storage system can come from a variety of sources and can have varying thermal quality. The cases considered in this study include a residential household, a 10- storey building and a retail/office space, all fitted with a concentrating solar panel array and an adsorption thermal energy storage system to fulfill the complete annual space heating needs. The panels supply heat to the building all year round. They have an excess of heat in the summer and can not supply the complete demand during the winter. The idea is to use the storage system to store the excess heat from the summer in the adsorption TES system and release it during the winter when space heating is required. The goal of this paper is to see if such a system is practically feasible. Prototype System An in depth description of the prototype system, adsorbent screening process and experimental procedure can be found in Dicaire 2009 [2]. The chosen adsorbent is a new hybrid adsorbent material produced by Rio Tinto Alcan in Brockville, Ontario, Canada. The adsorbent was placed in a stainless steel column and insulated. The adsorbent was exposed to humid air at varying flow rates and the energy released was determined with the use of thermocouples. A hygrometer was used to measure water input/outputs and to determine when the adsorbent was saturated. For regeneration, the adsorbent was exposed to hot air at varying temperatures until all possible humidity was removed. The energy required for the regeneration was measured. Adsorption Results & Regeneration Characterization The usefulness of an adsorption TES system depends on the energy released during the adsorption phase and the level of regeneration possible using the waste or solar heat. The focus of the research has been to determine the parameters that maximize the energy density during the adsorption phase and to characterize the level of regeneration based on the temperature of the available heat source. The system reaches temperatures as high as 70 o C and has a maximum energy density of 200kWh/m 3. It was found that if the system is well insulated, energy densities are fairly constant as a function of flow rate. It is possible to control the rate of energy released by manipulating the flow rate and relative humidity of the system. Lower flow rates and lower relative humidity cause slower release of energy. In these cases, the temperature of the outlet also diminishes and heat losses are greater, reducing the useful energy density of the system. This is why it is better to operate at 100% relative humidity and higher flow rates. 2

3 Figure 1 summarizes the regeneration characterization for the hybrid material. It shows the energy density achieved from different regeneration temperatures. The adsorbent bed was regenerated with hot air at various temperatures to remove the moisture and then put through a 100% relative humidity adsorption cycle to measure the amount of energy stored. Regeneration was performed at various temperatures (80 o C to 250 o C) and superficial velocities (0.15 m/s, 0.3 m/s and 0.45m/s). The regeneration temperature has a non linear relationship (NLR) with the energy density which is also displayed in the figure. Experiments were performed on clinoptilolite as well to confirm the regeneration trend seen in the hybrid adsorbent. Clearly, although the energy densities are not as high for this natural zeolite, the trend is identical and the model still applies. Repeat experiments were performed to obtain a standard deviation of the energy density which is represented on the graph by error bars. The regeneration flow rate was not found to have a significant impact on the energy density m/s 0.45 m/s 0.15 m/s NLR Hybrid adsorbent Energy Density (kwh/m 3 ) Clinoptilolite 50 NRL = ax b + x Regeneration Temperature ( C) Figure 1: Regeneration characterization of the adsorbents studied. The final piece of the regeneration characterization is the efficiency of the thermal storage. The energy released during the adsorption cycle is divided by the amount of energy used during the regeneration process to determine the percentage of energy stored, or efficiency of the thermal storage, which is displayed in Figure 2 as a function of the regeneration temperature for different superficial velocities. The efficiency varies linearly with regeneration temperature between 30% and 50%. The efficiency decreases as temperature increases, likely because of the additional heat losses from operating at a higher temperature. Experiments were performed at various superficial velocities and all 3

4 show similar trends. Therefore, between 30% and 50% of the thermal energy inputted to the system will produce useful heating Storage Efficiency m/s 0.45 m/s 0.2 m/s 0.15 m/s Regeneration temperature ( o C) Figure 2: Efficiency of hybrid adsorbent thermal energy storage system as a function of regeneration temperature for different superficial velocities. Feasibility Analysis The characterization of the adsorption TES from the previous section will be combined with a solar heat source and typical space heating requirement to complete the long term TES system. The analysis will explore three different scenarios; a residential household (250m 3 ), a 10-storey building (34 000m 3 ) and a retail/office space (14 000m 3 ). The solar heat source for the scenarios considered in this study are concentrating solar panels designed for thermal energy collection produced by Menova Energy Inc (Ottawa, Ontario, Canada) [10]: the residential PS-35 (21 kw thermal, span of 6.5m x 6.5m and a collector area of 35m 2 ) and the industrial PS-140 (85 kw thermal, span of 13m x 13m and a collector area of 140m 2 ). The space heating load has been modeled by HOT2000 V10.3 and EE4E, which are residential and commercial energy simulation software developed by the CANMET Energy Technology Center (Ottawa, Ontario, Canada) [5]. The residential household is combined with one PS-35 and the 10 Storey building and retail/office space are matched with several PS-140s. Figure 3 displays the monthly supply of energy by the PS-35 panel and demand for the household. The solid curve represents the space heating requirements of a typical Canadian household located in Montreal, Quebec. The other curve represents the average thermal output from the PS-35 including a 3% heat loss for piping. Similar trends are observed with the other two scenarios considered, although the magnitude of the curves is much greater. The 4

5 concentrating solar panels operate all year and can provide approximately 70% of the building space heating needs. They can supply the space heating needs alone for approximately 8 months of the year but require a complementary system for the remaining 4 months. The goal of the adsorption TES is to store the excess heat from the summer and use it for heating during the winter months to make up the missing 30%. Energy (kwh / Month) 3,500 3,000 2,500 2,000 1,500 1,000 PS-35 TH Thermal Output Typical Space Heating Requirement Months Figure 3: Monthly Energy Supply and Demand for a typical Canadian household located in Montreal, Quebec. The months where the solar panel outputs exceed the space heating needs shall be referred to as summer excess and the remaining months which require supplemental heat to fulfill the space heating needs shall be referred to as winter needs. The concentrators produce heat at approx 100 C for regeneration which means the TES has an energy density of 150kWh/m 3 and an efficiency of 50% (see Figure 1 and Figure 2). Table 1 summarizes the parameters of the complete adsorption TES for on-site use of excess thermal energy. When the energy required for regeneration of the TES (to store enough energy for the winter needs not supplied by the PS-35) is compared to the excess of energy from the summer, it is clear that there is enough energy. Table 1: Summary of Parameters for Adsorption TES Heat Source Excess Thermal Energy from Summer (kwh) Total Winter Needs not supplied by PS-35 (kwh) Energy Required for TES Regeneration (kwh) Bed Size (m 3 ) House 10 Storey Building Retail / Office PS-35 Thermal 5.5xPS-140 Thermal 5xPS-140 Thermal =(2.32m) = (10.11m) = (9.76m) 3 5

6 For the residential scenario, there is about 4 times more excess energy than required, which means that one PS-35 could supply more than one household with an Adsorption TES for winter space heating needs or even be used to also supply the Domestic Hot Water (DHW) needs. If we were to include DHW needs for the entire year, the PS-35 would still be able to deliver, but the adsorbent bed required would be 25 m 3. For the 10 storey building and the retail/office building, the excess summer heat is just enough to supply the regeneration of the TES and store enough energy for the winter needs. These large buildings are very likely to have enough area on their roof or somewhere else on their property to put the concentrating solar panels as well as the adsorbent bed necessary. It is possible to minimize the size of the adsorbent bed size by upgrading the regeneration heat from 100 C to 250 C by supplementary heating such as off-peak electricity or petroleum fuels. By increasing the regeneration temperature, the energy density is also increased which decreases the size of the adsorbent bed required to store the same amount of energy. However, this adds an operating cost for electricity or fuel usage. At 250 C, the energy density of the adsorbent is 200kWh/m 3 and has an efficiency of 30% (from Figure 1 and Figure 2). Upgrading with supplementary heat can reduce the adsorbent bed size by 25%. Although the size requirements for adsorption TES are still larger than conventional heating, it is a considerable improvement over the competing methods of thermal energy storage as can be seen in Figure 4 for the residential scenario. Storage Volume (m 3 ) Hybrid Adsorption (100 C) TES Water Sensible TES Wood Conventional Heating Hybrid (100 C) Hybrid (250 C) Water Rock <100 C PCM > 300 C PCM Wood Figure 4: TES size comparison for the proposed household being studied: Water 60kWh/m 3 [8], Rock 40 kwh/m 3 [8], <100 C PCM 56kWh/m 3 [1], >300 C PCM 300kWh/m 3 [4], Wood MJ/kg [7] Sensible storage systems, such as water, rock, or Phase Change Materials (PCMs) have very low energy densities and as a result require large volumes to achieve the same level of energy storage. High temperature PCMs have much higher energy density but require temperatures above 300 C, which is not useful for residential solar panels. It must also be recognized that energy storage through sensible TES degrades with time and would 6

7 not be useful for long term seasonal energy storage unless much larger volumes and extensive insulation were used. Also included on Figure 4 are the storage requirements for a year s worth of space heating with wood with a conventional wood furnace. The adsorption TES is not only much smaller and ideally suited for long term thermal storage compared with other TES, but its storage size is comparable to a conventional wood burning stove for space heating. Conclusions and Recommendations Adsorption TES systems are feasible for long term thermal energy storage and space heating applications. The prototype has proven that thermal energy can be stored and released from the adsorbent. Models for storage efficiency and energy density as a function of regeneration temperature have been developed through experimentation and are applied to determine the parameters of an adsorption TES for real life space heating applications. Although the size required for the sufficient energy storage is large compared to natural gas or electric heating, it is comparable to heating with a wood furnace and it is renewable and sustainable. Adsorption TES is much more practical than other competing forms of thermal energy storage. These types of systems could be applied to any residential, commercial or retail building. Since this TES system could also be portable, it can be applied for use of waste heat in commercial manufacturing and refineries or large solar farms and then transported to building which do not have the space for on-site solar energy collection. Using this approach, thermal energy could be delivered similarly to propane for space heating. Future experiments for this project will focus on improving the energy density of the adsorption TES in order to reduce the size of the storage bed necessary as well as the feasibility of upgrading the energy obtained from the power spars to achieve a higher level of regeneration. Future work will also include performing an economic analysis of the complete system to verify if implementation of a solar panel with adsorption TES is feasible and if it can compete with current heating applications. Possible uses for the remaining excess thermal energy produced from the power spars, such as supplying domestic hot water needs or space heating for a second home, will also be investigated. Acknowledgements The authors wish to thank the industrial partners that have made this publication and research possible: Menova Energy Inc, Rio Tinto Alcan and Natural Resources Canada. References 1 Akgün, M., O. Aydin, K. Kaygusuz, Experimental study on melting/solidification characteristics of a paraffin as PCM, Energy Conversion and Management, Vol 48, p , Dicaire, D., Masters Thesis in Preparation, Department of Chemical and Biological Engineering, University of Ottawa, Hauer, A., Evaluation of adsorbent materials for heat pump and thermal energy storage applications in open systems, Adsorption, Vol 13, p ,

8 4 Hoshi, A., D. R. Mills, A. Bittar, T. S. Saitoh, Screening of high melting point phase change materials (PCM) in solar thermal concentrating technology based on CLFR, Solar Energy, Vol 79, p , HOT2000 Software: /standard/hot2000.cfm?attr=4 6 National Energy Board, Canada s Energy Future, Scenarios for supply and demand to 2025, ISBN , 2003, 7 Ontario Ministry of Energy and Infrastructure, Heating and Cooling your home A conservative Guide, Shigeishi, R. A., C. H. Langford, B. R. Holleborne, Solar Energy Storage using chemical potential changes associated with drying of zeolites, Solar Energy Vol 23, p , Storch, G., G. Reichenauer, F. SCheffler, A. Hauer, Hydrothermal stability of pelletized zeolite 13X for energy storage applications Adsorption, Vol 14, p ,