Robert E. Critoph

Thermodynamic based comparison of sorption systems for cooling and heat pumping

Abstract A comparison of thermodynamic performances of sorption systems (liquid absorption, adsorption, ammonia salts and metal hydrides) is carried out for typical applications (deep-freezing, ice making, air-conditioning and heat pumping) with either air-cooled or water-cooled heat sink. The results are given in terms of cooling coefficient of performance (COP) (heating COP or coefficient of amplification (COA) for the heat pump), cooling (heating) power versus reactor volume or weight and thermodynamic efficiency. LiBr–water systems show the best results for air-conditioning except when small units are required (metal hydride systems lead to more compact units). Other systems, however, show better results for other applications (chemical reaction with ammonia salts for deep-freezing, adsorption for heat pumping).

Adsorption refrigerator using monolithic carbon-ammonia pair

Abstract This paper presents the description of a laboratory prototype of an adsorption cooling machine which uses an activated monolithic carbon-ammonia pair. The design is focused on the generator consisting of aluminium and monolithic carbon discs. A numerical model of the generator based on the mass and energy conservation equations is developed. The heat transfer model (between aluminium fin and activated carbon-ammonia pair) is validated by comparing computer simulation and experimental results at ice maker operating conditions corresponding to tropical use (the evaporating and condensing temperature ranges are −20°C to 0°C and 20°C to 45°C, respectively).

The use of psychrometric charts for the optimisation of a thermal swing desiccant wheel

Abstract An effective prediction is proposed to estimate the optimal rotation speed and performance of a rotary adsorber, in which simultaneous enthalpy and humidity changes are dealt with separately by visualising changes of state of product or exhaust air on a psychrometric chart. Assuming that the adsorbent rotor is completely regenerated to equilibrium with the regeneration air during the corresponding period, the optimal rotation speed corresponds to the region of the short time adsorption in which penetration theory holds and enthalpy exchange between both streams through the adsorbent rotor follows the behaviour of a rotary sensible heat exchanger at lower revolution rates. The change of the product/exhaust air condition with increasing rotational speed is presented as a set of simple equations. Also, by considering the relative humidity of product air and that of regeneration air to be almost the same at a sufficiently high flow rates of regeneration air, an optimal rotation speed and the product air condition are easily found by simple calculation. In comparison with experiments, the proposed method gives a rotational speed near the “optimum” and the humidity and temperature of the product air are predicted almost exactly.

Monolithic carbon for sorption refrigeration and heat pump applications

Abstract The thermophysical properties (effective thermal conductivity k, permeability K, porosity x and effective specific heat C) of two types of monolithic activated carbons are investigated with the intention of designing a high performance generator for sorption refrigeration systems and heat pumps using ammonia as refrigerant. This paper is mainly focused on the experimental procedures and results. Typical values obtained with one of the samples tested are: thermal conductivity = 0.44 W m−1 K−1, limiting concentration = 0.36 kg NH3kg−1 Carbon and carbon specific heat = 1080 J kg−1 K−1 at 100°C. The permeability results are highly anisotropic.

Forced convection adsorption cycles

Abstract The convective thermal wave is part of a patented cycle which uses heat transfer intensification to achieve both high efficiency and small size from a solid adsorption cycle. Such cycles normally suffer from low power density because of poor heat transfer through the adsorbent bed. Rather than attempting to heat the bed directly, it is possible to heat the refrigerant gas outside the bed and to circulate it through the bed in order to heat the sorbent. The high surface area of the grains leads to very effective heat transfer with only low levels of parasitic power needed for pumping. The cycle presented here also utilizes a packed bed of inert material to store heat between the adsorption and desorption phases of the cycle. The highest degree of regeneration possible leads to good COP. Thermodynamic modelling, based on measured heat transfer and porosity data, predicts a cycle COP (for a specific carbon) of 0.95 when evaporating at 0°C and condensing at 42°C. These temperatures are compatible with ARI conditions. Further improvement is possible. Experimental heat transfer measurements and cycle simulations are presented which show the potential of the concept to provide the basis of a gas-fired air conditioner in the range 10–100 kW cooling. A research project to build a 10 kW water chiller is underway. The laboratory system, which is being commissioned at the time of writing, is described.

Evaluation of alternative refrigerant—adsorbent pairs for refrigeration cycles

Abstract The refrigerant-adsorbent pairs at present preferred for solid-sorption refrigeration cycles are ammonia-carbon, methanol-carbon, ammonia salts and water-zeolite. Porosity tests have been carried out on a range of alternatives, including R32 and butane, as refrigerants and new monolithic carbon adsorbents. The results of fitting to the Dubinin equation and modelling of cycles based on these pairs are presented. Ammonia is still preferred on efficiency grounds but R32 may have applications where the toxicity and incompatibility of ammonia and copper prevent its use.

Activated carbon adsorption cycles for refrigeration and heat pumping

Adsorption cycles were used for cooling before the advent of reliable mechanical compression refrigerators, and are now attracting renewed interest as the basis of solar powered refrigerators for vaccine storage etc. They also have potential for use in heat pump systems. The cycle thermodynamics are reviewed and the suitability of a range of refrigerants with active carbon adsorbents is assessed. Methanol, sulphur dioxide, ammonia, methyl amine, and formaldehyde are of particular interest. Experimental p-T-x data is presented for ammonia and methanol with a range of carbons, together with their calculated performance in refrigeration cycles. Currently available carbons can give good performance, but it is possible that specially developed carbons might show significant improvement. A major development area lies not in changing the porosity characteristics but in trying to improve heat and mass transfer to and from the adsorbent.

Simulation of a continuous multiple-bed regenerative adsorption cycle

Abstract 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 3 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.

Performance estimation of convective thermal wave adsorption cycles

The convective thermal wave adsorption cycle is described and a thermodynamic model which takes account of the variations in temperature and concentration in the bed is proposed. The results of using the model with a particular carbon adsorbent and ammonia adsorbate are presented for a range of cycle temperatures and system heat transfer effectiveness values.

Multiple bed regenerative adsorption cycle using the monolithic carbon–ammonia pair

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.