C.L. Heard

Thermodynamic study of advanced absorption heat transformers. I: Single and two stage configurations with heat exchangers

A thermodynamic analysis was carried out to study the effect of heat exchanger effectiveness (EF) on the performance of single stage heat transformers (SSHT). Moreover, an analysis of three different arrangements of two stage heat transformers was performed using a mathematical model assuming water/lithium bromide as the working fluid. An increase in the solution heat exchanger effectiveness (EF) greatly improved the performance of absorption heat transformers when the absorber temperature was at least 40°C higher than the temperature of the heat supplied to the system. In two stage heat transformers (TSHT), higher absorber temperatures were obtained by coupling the absorber of the first stage to the evaporator of the second. However, higher performance coefficients were obtained in general by coupling the absorber of the first stage to the generator of the second.

Thermodynamic study of advanced absorption heat transformers—II. Double absorption configurations

Abstract A thermodynamic analysis has been carried out to study the performance of double absorption heat transformers (DAHT) assuming water/lithium bromide as the working fluid. The performance of single (SSHT) and two stage heat transformers (TSHT) analyzed in Part I, was compared with the performance of double absorption heat transformers (DAHT) under the same operating conditions. The results showed that single stage heat transformers (SSHT) were the simplest and most efficient. Greater absorber temperatures were reached with two stage heat transformers (TSHT). However, these systems were in general less efficient than the others and technically the most complex. Double absorption heat transformers (DAHT) were technically simpler than two stage heat transformers (TSHT) and may reach absorber temperature as high as these systems.

Experimental performance of ternary solutions in an absorption heat transformer

In this work, results from experiments with ternary solutions in an absorption heat transformer are presented. The experiments were performed under controlled conditions using water/lithium chloride/zinc chloride and water/calcium chloride/zinc chloride solutions as working pairs. The results showed that the gross temperature lift is increased with regard to the results obtained using binary solutions because the concentration of the solutions was enhanced. The water/lithium chloride/zinc chloride solution showed a generally better performance than the water/calcium chloride/zinc chloride mixture. The highest gross temperature lift for the former solution was 37.5°C for an absorber temperature of 96°C. This result compared favourably to that previously obtained for water/lithium bromide in the University of Salford.

Performance study of a double-absorption water/calcium chloride heat transformer

In order to increase the temperature lift and efficiency of single-absorption heat transformers there are other possible arrangements. Double-absorption heat transformers have a relatively simple design and smaller size compared to two-stage heat transformers. In this work, the thermodynamic performance of the water/calcium chloride system was modelled for a double-absorption heat transformer. Results indicate that temperature lifts of up to 40°C are possible with coefficients of performance close to 0·3.

Experimental performance of the system water/magnesium chloride in a heat transformer

Absorption heat transformers are devices with the unique capability of raising the temperature of part of a low grade heat source whilst simultaneously rejecting the rest of the heat at a lower temperature. The gross temperature lift that could be attained in the process depends on the characteristics of the working pair. Many combinations of working fluid/absorbent have been proposed although until now the water/lithium bromide system is the most widely used. Experimental results for the water/magnesium chloride working pair in an absorption heat transformer are presented. Two different ranges for the absorber temperature were investigated. The absorber temperature varied from 81 to 89°C and from 91 to 101°C. For the first case, the gross temperature lift was calculated between 7⋅8 and 10⋅2°C whilst for the second case the gross temperature lift was found to be between 15 and 18⋅4°C. For both sets of experiments, the heat input was maintained constant and the calculated coefficient of performance was related to the absorber temperature, the flow ratio and the effectiveness of the economizer.

Development in geothermal energy in mexico—part twenty seven: The potential for geothermal organic rankine cycle power plants in Mexico

Mexico possesses large amounts of geothermal energy. Samples from over 800 geothermal surface phenomena indicate that only 1.1% have an estimated reservoir temperature greater than 200°C. Current practice in Mexico is to produce power from such reservoirs using an open flash steam cycle. It is estimated that 4.8% of geothermal resources are in the temperature rangefrom 140 to 200°C which is a suitable range in which to operate Organic Rankine cycle power plants. Organic Rankine cycle power plants have been built in a range of sizes from 10kWe to 45 MWe. They have considerable potential for increasing the production of electricity from Mexicos geothermal resources. Organic Rankine cycle plants are discussed together with the conditions for their economic operation.

Developments in geothermal energy in Mexico—Part twenty six: Experimental assessment of an ammonia/water absorption cooler operating on low enthalpy geothermal energy

Abstract Mexico possesses large amounts of geothermal brine at temperatures which are too low to enable electricity to be generated efficiently and economically. Most of the geothermal fields in Mexico are located near important agricultural areas. Perishable food losses in Mexico, resulting from inadequate handling and cold storage facilities, vary from 35 to 50%. In order to prove the technical feasibility of operating heat-driven absorption cooling systems on low enthalpy geothermal energy, a prototype ammonia/water absorption cooler was installed in the Cerro Prieto geothermal field, where the ambient temperatures exceed 40°C and the cooling water temperatures reach 30°C. The unit has operated successfully with evaporative cooling loads exceeding the design value. The experimental data obtained will provide an excellent basis for the design of large scale heat-driven absorption refrigeration systems.

Developments in geothermal energy in Mexico—part forty. The future for geothermal energy in Mexico

Abstract In Mexico there are many potential geothermal energy sources with a wide range of temperatures. At present high temperature geothermal energy is used to generate electricity. Considerable experience has been accumulated in the exploitation of geothermal energy. Lower grade geothermal heat could be used to generate electricity with organic Rankine cycle plants. There is little industrial exploitation of lower temperature geothermal resources at present even though in the centre of Mexico areas of geothermal activity coincide with industrial zones. However, this situation could change with the use of suitable heat exchangers and heat pumps, particularly absorption systems to supply process heat with very little pollution.

Developments in geothermal energy in Mexico—part sixteen. The potential for heat pump technology

Abstract Mexico possesses large amounts of geothermal brine at temperature which are too low to enable electricity to be generated efficiently and economically. Any system which extracts useful energy from a geothermal source is limited by the effectiveness of the heat transfer between the geothermal fluid, which has a tendency to scale, and the relevant components of the system. A heat pump can be used to maintain a temperature difference between two vessels containing pure water and geothermal brine, which is sufficient to enable pure water vapour to flow continuously from the geothermal brine vessel to the pure water vessel. The hot water produced can then be used to operate an absorption cooling system which can be used to store food. Alternatively a heat pump can be employed to increase the temperature of the hot water to produce low pressure stream.