Absorption Refrigeration with Thermal (Ice) Storage
Description of the project
This proposal is concerned with the manufacture and testing of a novel thermal ice storage system for use with absorption refrigerators powered by waste heat from combined heat and power systems.
It is now recognised that the combination of CHP with absorption refrigeration for District Heating and Cooling schemes can lead to reductions in electricity demand when waste heat from the CHP unit is used to provide space cooling. It is believed that the addition of Thermal (Ice) Storage systems to such schemes could provide a means of smoothing loads on district 'cooling' networks. This could further reduce the installed electricity generating capacity of CHP plant by removing the need for electrically powered air chillers, whilst reducing the installed cooling capacity of the absorption plant and therefore reducing capital cost whilst saving energy.
It is well known that Thermal Ice Storage (TIS) is successfully used with electrically powered vapour compression refrigerators in building cooling systems and with water-ammonia absorption refrigerators in industrial applications. However, the use of water-ammonia absorption plant in building cooling air conditioning systems is generally avoided due to fire and toxicity hazards. The industry standard at this time for absorption plant is lithium-bromide-water (LiBr/H2O). So far no one has contemplated using TIS technology with lithium-bromide-water systems for the reason that to make ice requires a saturated refrigeration temperature at the evaporator of less than at least -5oC and of course the refrigerant, in this case water, itself freezes at 0oC. An efficient means of producing water-ice for TIS using an otherwise conventional LiBr/H2O absorption refrigerator system has been developed. The method proposal here is novel and untried, however, preliminary studies have been carried out to investigate areas of uncertainty and preliminary results are have been very encouraging. From the results of these preliminary studies there is no reason to believe this innovative project will not be successful and eventually find useful commercial applications.
To undertake a proof-of-concept manufacture and testing programme to assess the practical feasibility of the absorption cycle thermal ice-store concept over a range of evaporator, generator and absorber operating conditions.
An existing single-effect vapour absorption cycle refrigerator will be modified to include the thermal (ice) store and ejector. The existing refrigerator includes an 8kW(max) LiBr concentrator, powered by a 10kW/15bar(max) steam generator, a 8kW(max) water/LiBr absorber and a 10kW condenser. The rig is electronically controlled and has a range of safety cut-outs. It is fully instrumented for temperatures, pressures and flows and has a computerised data acquisition system.
A 100 litre ice store will be manufactured which is based on the results taken from the preliminary experimental work. It is proposed to base the absorption cycle refrigerator on a current machines which is powered by a 10kW electric heater. A single ice-store vessel will be manufactured along with a suitable ejector. It is anticipated that the manufacture of the ice-store, the modification of the absorption rig and it commissioning will take approximately 12 months.
Summary of the final report of the project
This report describes and evaluates the results of an experimental study of an innovative absorption cycle refrigeration and thermal (ice) storage system. The report describes the manufacture and testing of a laboratory scale lithium-bromide/water single-effect absorption cycle refrigerator machine and a novel thermal (ice) storage unit.
Experimental data confirming the technical feasibility of the novel system is provided and recommendations for future research and a route to exploitation are also discussed.
The investigation included experiments to determine:
- Optimum concentration of water-gel used in the construction of the storage elements.
- Optimum layout of the elements within the TIS.
- Entrainment and pressure lift performance of the jet-pump.
- Optimum location of the primary nozzle within the mixing chamber.
- Critical back-pressure of the jet-pump.
- Freezing rate of the TIS.
- Coefficient of performance of the cycle.
The results demonstrated that the absorption refrigerator operated satisfactorily, however, there are some minor modifications needed to ensure a more stable operation. Some improvements to the instrumentation are also required.
Before going forward to develop the concept further it will be necessary to define in some detail the capital and life-cycle costs of such a plant compared with a standard absorption machine. This study is required in order to quantify the potential cost and energy savings resulting from the potential downsizing the absorption cycle machine and the cost of the TIS system. Clearly, the anticipated decrease in capital costs would boost the economic feasibility of future applications.
The study has demonstrated the technical feasibility of the novel TIS - absorption cycle concept which is thought to offer the potential to promote the wider use of Combined Cooling Heating and Power (CCHP) schemes. However, to progress further it will be necessary to involve industrial partners.
School of the Built Environment,
University of Nottingham, United Kingdom