R.E. Critoph
University of Warwick, School of Engineering, Coventry CV4 7AL, UK

A refrigeration / heat-pump system based on a number of simple tubular adsorption modules is described. A single module is comprised of a generator and a receiver/condenser/evaporator. A single generator consisting of a 12.7 mm stainless steel tube lined with 2.6 mm of monolithic active carbon has been manufactured. A complete module has been tested in a simple rig, which subjects it to alternating hot and cold airstreams, desorbing and adsorbing ammonia. A complete system, consisting of 32 modules has been modelled in detail and its predicted performance is presented. Key parameters have been varied and their effect on the performance discussed.

W. Wang and R.Z. Wang
Institute of Refrigeration & Cryogenics, Shanghai Jiao Tong University, Shanghai, 200030, P.R. China

There is abundant waste heat in the fume exhausted from diesel and gasoline engine, which is a potential candidate to drive a adsorption refrigeration system. Generally, the temperature of fume is high enough, but its convective heat transfer coefficient to adsorber and the thermal conductivity of adsorbent are low. The capability of heat transfer should be taken into account in designing an adsorber for refrigeration. However, the structure and operating parameters, related to heat transfer, are connected each other, their optimum values for the performance of refrigeration system should be elected according to their relation and operation condition. This paper is try to provide some reference consideration in design an adsorption refrigeration system.

Z. Tamainot-Telto and R.E. Critoph
University of Warwick, School of Engineering, Coventry CV4 7AL - UK

ln order to design and build a low cost rotary regenerative adsorption air conditioning system using monolithic carbon-ammonia in a multiple bed design, various single module configurations have been tested. The basic single module named MODUIAR1 consists of stainless steel tube 12.7 mm in diameter, wall thickness 0.25 mm and 600 mm long and contains about 2.6 mm layer of monolithic carbon (about 40 g) from WaterLink Sutcliffe Carbons Ltd. The far end of the module (200 mm long) is the evaporator-condenser (ammonia liquid receiver). An inert material (PTFE) is inserted between the receiver and the generator as an adiabatic section that reduces the longitudinal conduction between them. From the basic module, a second module configuration named MODULAR2 is considered with external aluminium fins to improve the heat transfer capacity of the module: it consists of two basic single modules with both generator and receiver fitted with 0.3 mm thick rectangular aluminium fins (55 mm x 27.5 mm). This paper is focused on detailed design, construction and testing of both modules (MODULAR1 and MODULAR2). The experimental results are presented and discussed. With a generating temperature of 100°C, a condensing temperature of about 33°C and an evaporating temperature varying from -5°C to 20°C, MODULAR2 has provided a maximum specific cooling of about 0.410 kW/kg carbon while MODULAR1 has produced up to 0.240 kW/kg carbon. The typical COP of both modules is about 0.20. A numerical model of the two modules is validated by comparing experimental data with simulation predictions. The heat transfer coefficients identified are about 80 W/m² K and 240 W/m² K for MODULAR1 and MODULAR2 respectively.

(co)authored by : G. Restuccia, A. Freni and G. Maggio
CNR - Institute for Transformation and Storage of Energy, S. Lucia sopra Contesse 5, 98126 Messina, Italy

In this paper the heat and mass transfer properties of a new zeolite-coated adsorbent bed to be employed in sorption air conditioning systems are investigated by a modelling approach. It consists of a dynamic model which allows to calculate the exchanged energies, the cycle time and, thus, the specific power of the bed. The analysis of the model results, has shown that the proposed configuration, in which the heat transfer enhancement is mainly related to the good adhesion between metal and adsorbent, is very interesting if compared with the traditional beds. Furthermore, an optimisation study has been carried out to determine the conditions which allow to obtain the most effective heat and mass transfer in the new adsorbent bed.

R.Z. Wang, Y.X. Xu, J.Y. Wu, M. Li, H.B. Shou
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, China

A combined cycle capable of heating and adsorption refrigeration is proposed, and the experimental prototype has been installed. The system consists of a heater, a water bath, an activated carbon-methanol adsorption bed and a ice box. This system has been tested with electric heating, and has been found that with 61 MJ heating, the 120 kg water in the bath can be heated up from 22^oC to 92^oC meanwhile 9 kg ice of -1.5^oC is made. The calculated COP_system is 0.0591 and COP_cycle is 0.41. After reconstruction to a real hybrid household water heater-refrigerator, when 55MJ heating is added to 120kg 21^oC water, and the condensing temperature is controlled at about 30^oC, the result is the 4kg water contained inside the methanol refrigerant evaporator was iced to -2^oC, the cooling capacity of the ice and the refrigerant in the evaporator will maintain the 100 liter cold box for about three days below 5^oC. The experiments show the potentials of the application of the solar powered hybrid water heater and refrigerator. Theoretical simulation has been done, which is in good agreement with experimental results. This research shows that the hybrid solar water heating and ice-making is reasonable, and the combined cycle of heating and cooling is meaningful for real applications of adsorption systems.

R. Thorpe
School of Engineering, University of Warwick, Coventry, CV4 7AL United Kingdom

A solid adsorption heat pump has been built using the refrigerant gas itself as a heat transfer medium. A moving temperature front or 'thermal wave' is created within the bed of granular active carbon to facilitate a highly regenerative cycle. A novel and compact geometry has been used to simplify manufacture and to reduce the volume of the machine.

J.Y. Wu, R.Z. Wang, Y.X. Xu
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai (200030), P.R. China

Heat recovery cycle plays an important role in increasing operation performance of a continuous heat recovery adsorption heat pump. The real heat recovery ratio would be less than the idea heat recovery ratio because of limited heat transfer coefficient of the adsorber. In this paper, by the dynamic calculation of a continuous heat recovery adsorption heat pump, heat recovery ratio in different working conditions was determined. At the meanwhile the influence of adsorber heat transfer coefficient was analyzed, and the influences of system operation parameters, such as heat source temperature, cooling water temperature, cycle time and so on, on heat recovery process were also analyzed. How to increase usable heat recovery capacity was discussed. This work has laid a foundation for further analysis of heat recovery cycle.

A. Hauer
ZAE Bayern, Center for Applied Energy Research, Domagkstr.11, 80807 Munich, Germany

Zeolites adsorb water vapor in an exothermic reaction [1]. Adsorption and desorption of water vapor on Zeolites can be used for thermochemical energy storage using low temperature heat [2]. Based on the results of a pilot system [3] an energy storage with 7000 kg zeolite 13X was installed in a school building in Munich, Germany, by 1996 and connected to the district heating system. The storage is charged by heat during off-peak periods at desorption temperatures of 130°C – 180°C. During peak time the heating system of the building can be disconnected from the district heating system and is powered by the energy stored in the zeolite, thus reducing peak power demand of the district heating system. The use of storage systems like this is raising the efficiency of the district heating system by leveling out the power demand. The automatic operation of the storage system started in 1997. An energy density of 124 kWh/m³ and a thermal coefficient of performance COP_th up to 0.92 could be experimentally obtained. This correlates to 81%, 86% respectively, of the theoretically calculated values under the same conditions. The application of the system as a desiccant cooling device is currentlyunder investigation in a R&D project.

S. Waszkiewicz, H. Saidani-Scott, M. Tierney
Mechanical Engineering Department, University of Bristol, Queens Building, University Walk, Bristol BS8 1TR, UK

Environmental protection initiatives by environmental agencies are necessitating the replacement of chlorofluorocarbons with benign working fluids. One of the sensitive areas affected is refrigeration and heat pump technology, where new working pairs are being developed as an alternative to the traditional CFCs. This will have less impact on the destruction of the ozone layer.
In the design of adsorption refrigeration and heat pump systems, it is important to analyse precisely the performance of the cycle. This is based on an accurate determination of the adsorbent-adsorbate performance. Therefore, the thermodynamic behaviour of adsorbent materials has to be studied in detail using a number of physical models, which are widely accepted. Various kinds of sorption systems have been developed, mostly of activated carbon-ammonia, activated carbon-methanol, silica gel-water and zeolite-water pairs
A new refrigeration system, studied at Bristol, working with zeolite and methanol is presented. The following features are novel, (a) integration of heat transfer and adsorption via a finned surface coated with zeolite CBV 901 and (b) the use of connected, twin active bed system to enable heat recuperation. A thermal model based on Dubinin-Astakhov equation and thermodynamic analyses, which presents a complete and precise explanation of adsorption-desorption cycle, is given. The model description considers the effective specific heat dq/dT depends at any point in the cycle. Thus, dq/dT was found to depend on the thermal properties of the zeolite, heat of adsorption, and the amount of fluid adsorbent per unit temperature change. Suitable interpretation of dq/dT has produced acceptable heat input, heat output and an optimistic Coefficient of Performance (COP).
In parallel, computer simulations to determine the effects of operating conditions on cooling output and COP conduct a parametric study. The analysis confirms that large changes on COP are possible. Typical estimates are 0.535 for a single bed cycle and around 0.935 for a twin bed cycle.

Y. Kato, A. Minakami and Y. Yoshizawa
Research Laboratory for Nuclear Reactors, Tokyo lnstitute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan

The operability of a chemical heat pump using magnesium oxide/water reaction system was discussed experimentally under hydration operation pressures between 30 kPa and 203 kPa. The heat pump was expected to be applicable in cogeneration systems using gas and diesel engine, or fuel cell and micro gas turbine. In the experiment, a reactant having high durability for repetitive operation was packed in a cylindrical reactor. The heat pump was operated thermally with no mechanical work. The operation of a unit cycle consisted of the endothermic dehydration of magnesium hydroxide for heat storage, and the exothermic hydration of magnesium oxide for heat release. The cycle of operation was repeated under various thermally driven operation conditions. The forward and reverse reactions were studied by measuring the reactor bed temperature distribution and the reacted fraction changes. The reactor bed could store heat around 300-400°C by the dehydration reaction and release heat around 100-200°C by the hydration reaction under heat amplification mode operation. The practical possibility of the reactor bed was discussed based on the experimental results. The heat pump was expected to be applicable for a load leveling in a cogeneration system by chemical storage of surplus heat at low heat demand and by supplying heat in the peak load period.

H.P. Klein, M. Groll
Institut für Kernenergetik und Energiesysteme (IKE), Universität Stuttgart
Pfaffenwaldring 31, D-70550 Stuttgart, Germany
Tel. (+49) 711-685-2127, Fax (+49) 711-685-2010,
e-mail : h.klein@ike.uni-stuttgart.de

Thermally driven sorption heat pumps compete to be an alternative to mechanically driven vapour compression heat pumps. They do not use CFC refrigerants and therefore have no Ozone Depletion Potential (ODP) and only a negligible Global Warming Potential (GWP). However, their perfomance lacks behind, even if in the case of compression devices the efficiency of electricity generation is taken into account. The currently available sorption devices have a coefficient of performance for cooling (COP) of about 0.75 for single-effect systems and of 1.2 for double-effect systems. Since the temperatures and pressures under which sorption systems are operated differ widely, it has been suggested to combine soprtion systems operating with different working pairs to form a cascading system, in which a topping cycle is producing cold and heat at a sufficiently high temperature level to be able to drive a bottoming cycle which also produces cold, thus increasing the COP.
In this study, a two-stage metal hydride sorption device is investigated, which is used as a topping cycle in a cascading system. The system comprises the 3 metal hydrides LmNi_4.85 Sn_0.15, LaNi_4.1Al_0.52Mn_0.38 and Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 in two reactors each. It is operated with a driving temperature of 310 °C, releasing heat for driving a bottoming cycle at a temperature of 125°C and producing cold at a temperature of 2 °C. With a half-cycle time of 15 min and using the reaction enthalpies and the exchanged amount of hydrogen, the heat and cold output of the system can be determined. The total cold production is 2 kW and the heat generation is 1.7 kW. Therefore, the COP is in the range of 0.9 and the coefficient of heat amplification COA around 0.8. If a double-effect lithium bromide-water system with the above mentioned COP is used as the bottoming cycle of the cascading system an overall COP of 1.8 to 2 is expected.