DEVELOPMENT OF SOLAR THERMAL HEATING AND COOLING SYSTEMS USING THE JET PUMP CYCLE

Transcription

1 DEVELOPMENT OF SOLAR THERMAL HEATING AND COOLING SYSTEMS USING THE JET PUMP CYCLE R.FENTON (a), S.KNOWLES, I.W.EAMES, G.MAIDMENT (a) Industrial Design Consultancy Ltd, Unit 13 The Portland Business Centre, Manor House Lane, Datchet, Berkshire, SL4 3EQ, United Kingdom Fax : +44(0) , ryan.fenton@idc.uk.com ABSTRACT The jet pump or ejector cycle is a proven, thermally-driven cooling circuit. This paper describes the development of a refrigerator and air-conditioning (AC) system adapted to run from solar thermal panels. The development and performance testing of a small scale (1.5kW) AC system and a household sized refrigerator (80W) is described. Based upon achieved cooling performance of these systems, a design for a modular, solar thermal, heating and cooling system is proposed. The operation of the proposed system is explained and predictions are given for the heating and cooling performance and for the energy saving potential of the system. 1. INTRODUCTION Cooling demand increases with a rise in ambient temperatures, hence using solar energy to power cooling devices provides a synergistic solution to meeting this demand. Considering central air conditioning (AC) systems, one estimate shows that between 1980 and 2000 the European demand increased approximately 300% (Marchio, 2008). With the industrialisation of developing nations, it is likely that we will see a worldwide increase in such cooling requirements with an associated rise in energy costs. Studies estimate between 40-60% of world energy demand is required for heating and cooling duties (Seyboth et al., 2008) (Cogeneration Technologies, 2008). Presently, only 2-3% of this demand is met using renewable energy technologies (Seyboth et al., 2008). Despite the availability of up to * 1kW m -2 solar energy, solar powered cooling systems have a worldwide installed capacity of 50MW (Seyboth et al., 2008). Various technologies have been developed to try and harness this energy and these can be divided into two categories; solar electric and solar thermal. Solar electric systems have already been developed which normally employ vapour compression technology. The drawback of such systems is that they require photovoltaic cells, batteries and charge controllers leading to prohibitively high costs. Assuming a panel efficiency of 10% and coefficient of performance of 3.0, PV refrigeration systems of this type cost around 4,200 per kw (Kim and Infante Ferreria, 2008). Solar thermal technologies offer an attractive alternative due to higher panel efficiencies and lower costs. In addition, these systems can be powered by a number of different thermal energy sources including solar, biomass, geothermal or waste heat. Research into solar thermal cooling has been primarily focussed on sorption systems. Adsorption systems, which have been widely researched, range in cost between 1,000 and 1,200 per kw (Kim and Infante Ferreria, 2008). Jet pump technology provides a simple and reliable alternative to provide cooling from a thermal energy source. This paper describes how the jet-pump cycle can be used to power both refrigeration and AC units from a solar thermal panel array with particular reference to the coefficient of performance (COP). COP is widely used as a measure of efficiency for thermodynamic systems. For a thermally driven cooling system it can be defined as; COP cool = Q e / Q g (1) Where Qe, is the cooling power of the evaporator and Qg is the heat input. It should be noted that COP cool does not take into account either the panel efficiency or the feed pump/fan electrical requirement. For reference, the panel efficiency is assumed to be 0.5 and the pump power requirement to be less than 150W. * Peak value for a sunny day which exists for no more than 4 hrs per day 1

2 2. JET PUMP TECHNOLOGY The jet-pump, or ejector, was originally invented at the start of the twentieth century to remove air from steam engine condensers. Although large AC systems using jet-pumps were developed during the 1930 s the technology has received relatively little development attention since (Chunnanond and Aphornratana, 2004; Eames et al., 1995). It is only during recent times that the potential benefits of the technology have been revisited. A jet-pump uses a high-pressure source, known as the primary flow, to create suction and entrain vapour or fluid, known as the secondary flow. The primary flow enters the jet-pump via a convergentdivergent (De Laval) nozzle and is accelerated to supersonic speeds, as shown in figure 1. As the flow exits the nozzle, an area of low pressure is formed within the mixing chamber and secondary flow is entrained and carried into the diffuser section of the jet-pump. Figure 1 - Jet-pump illustrative diagram and flow plot The jet-pump can be used to replace the electrically powered compressor within a traditional vapour compression refrigeration circuit, as shown in Figure 2. Thermal energy is used to provide heat to a vapour generator. This vapour is then used as the primary flow to entrain fluid from the evaporator and generate the cooling effect. The jet-pump therefore acts as a thermally powered compressor. Figure 2 - Simplified jet pump cooling schematic The entrainment ratio is one of the key performance characteristics of any jet-pump and is calculated using the following formula. Entrainment ratio, R m = secondary mass flow / primary mass flow (2) 2

3 The secondary mass flow determines the cooling capacity of the system and hence the COP cool of a jet-pump refrigerator is closely related to the entrainment ratio (R m ). Under normal operation, the supersonic flow is choked within the constant area section of the diffuser. The position of this effect is governed by the pressure at the outlet of the diffuser section. As the outlet pressure increases, the choking location moves backwards into the mixing section and the entrainment ratio decreases rapidly. The outlet pressure at which this occurs is known as the critical value (Eames et al., 1995). Operating close to the critical condenser pressure ensures that the jet pump is performing at an optimum level (Chunnanond and Aphornratana, 2004). The only way to increase the critical value is to increase the pressure of either the generator or the evaporator. This requires either more heat input or a higher minimum cooling temperature respectively. Various studies have also shown that the position of the nozzle is critical to the operating performance of the jet pump (Selvaraju and Mani, 2006). Retracting the nozzle out of the diffuser generates a higher entrainment ratio, and higher COP, however reduces the critical condenser pressure. Conversely, higher condenser pressures can be achieved by inserting the nozzle further inside the diffuser. Working close to the critical value helps to achieve the best possible performance and therefore an optimum nozzle position must be chosen for each system. A literature review of jet-pump cooling research was conducted to evaluate the state-of-the-art. Various working fluids have been trialled for jet-pump systems and in particular the use of water has been widely researched (Chunnanond and Aphornratana, 2004; Eames, 2006; Eames et al., 1995; Sun, 1999). Using water as a refrigerant offers several benefits, however it s thermodynamic properties also act as limitations. Water systems require higher generator pressures to achieve similar COP s when compared to hydrocarbon refrigerants (Chunnanond and Aphornratana, 2004; Eames et al., 1995; Selvaraju and Mani, 2006; Sun, 1999). Various theoretical and experimental studies of jet-pump driven cooling systems that use hydrocarbons have been evaluated as part of the review. Amongst the HFC s evaluated, R134A was shown to be a suitable working fluid, achieving theoretical COP s ranging from 0.2 (Alexis and Karayiannis, 2004) to 0.46 (Sun, 1999) for generator temperatures under 100 o C. Experimental studies have also proven that COP s in excess of 0.2 can be achieved using R134A. (Selvaraju and Mani, 2006). Although research has previously been conducted on jet-pump refrigeration systems, the work in trialling / commercialising such technologies has not been fully explored. In order to analyse the potential of jet-pump refrigeration fully the system must be refined and optimised and this is the subject of this paper. 3. JET PUMP REFRIGERATOR PROTOTYPE A small scale, portable refrigerator prototype was designed, rated according to the energy available from 2m 2 of solar thermal panels. At times of high irradiance, when cooling requirement is greatest, the collectors would be capable of delivering 1kW assuming a panel efficiency of 0.5 and a solar irradiance of 1kW m -2. Based upon previous research reviewed work and the availability of suitable components, R134A was identified as the most suitable refrigerant. A CFD study was conducted and a COP cool of 0.1 was targeted. Based upon the panel rating, predicted COP cool and thermodynamic properties of R134A the values, described in table 1, were defined for each system component. Table 1 - Refrigerator prototype design-point conditions Location State Temp ( o C) Abs Pressure (MPa) Enthalpy (kj kg -1 ) Power W Generator Liquid ,000 Generator Vapour " " " Condenser Liquid ,100 Evaporator Vapour A jet pump assembly was precision manufactured in brass that allowed incremental adjustment (0.25mm) of the primary nozzle position. Following system commissioning, experimental results were used to determine the optimum nozzle position for maximum entrainment ratio at the design point condenser temperature. The results from these optimisation tests are detailed in the figure 3 below. 3

4 12 Minimum temperature in evaporator ( o C) Nozzle spacing from diffuser (mm) Figure 3 Graph showing minimum evaporator temperature against nozzle position The cool-box was constructed in MDF with Styrofoam insulation and a polystyrene liner. The conductance was experimentally found to be 1.38W K -1. The temperature of the cool-box air, pipe-work and primary load (24 litres of water) were monitored during operational tests were used to determine the power rating of the evaporator via the formula Q = m.cp.δt. A photo of the prototype system is shown in figures 4 & 5 below. Figure 4 Photo of refrigerator prototype Figure 5 Photo of refrigerator cool box The vapour generator, used to power the jet-pump with high-pressure refrigerant vapour, was designed and constructed using a brazed plate heat exchanger. For operational tests an oil heater was used to simulate the heat input available from a 2m 2 solar collector. Following flow rate calibration of the pump, the generator input power could be calculated instantaneously using the temperature difference across oil input/output. A generator fluid delivery system based upon a thermally driven concept was designed and manufactured that provided operation based around electronically controlled charging/discharging cycles. Various tests were conducted to determine the optimum cycle period. Check valves were installed to prevent reverse flow of the refrigerant during charging cycles. Following initial verification of the operating procedure, electrically actuated valves were installed and the system was automated using a programmable logic controller (PLC). The PLC collected data from pressure/temperature transducers and an opto-electronic sensor to monitor the system condition and control the flow of refrigerant accordingly. Pressure relief valves, pressure sensors and thermostatic controls were also installed to ensure safe operation of the system during tests. The PLC provided the stable platform to enable the collection of reliable data sets over extended periods. 3. REFRIGERATOR PROTOTYPE RESULTS AND DISCUSSION Results from one of the ten hour tests are shown in figure 6 below. This shows the system successfully cooling 24 litres of water from 21 o C to 10 o C. The initial performance of the unit compared favourably providing over 110W of cooling, however as the temperature within the cool-box dropped, the COP cool fell below the 0.1 design point. The reason for the drop off in performance was apparent at lower evaporator 4

5 pressures as the critical condenser pressure decreased and flow was choked within the mixing section of the diffuser. Installing additional jet-pumps or a variable geometry jet-pump could provide a solution to this problem however these measures have associated cost implications. Figure 6 Jet-pump refrigerator performance over 10hr test period 5. JET PUMP AIR CONDITIONER (AC) PROTOTYPE The experimental results collected from operational tests on the jet-pump refrigerator showed the potential for achieving design point cooling however both the small size of the system and low temperature of the evaporator proved to be limiting factors. The experimental results confirmed that higher COPs could be achieved with higher evaporator pressures hence indicating the system may be more suited to an AC application. In addition an increase in cooling power would require a larger jet pump that would be less expensive to manufacture The basic cooling cycle of the AC system remains the same as the circuit described in figure 2, however a new refrigerant R245fa was selected to provide a closer match to the operating conditions of the system. Due to the increase in duty, most of the primary components of the system needed to be resized. The design point parameters of the system were defined as follows. Table 2 Jet pump AC system parameters Location Temperature ( o C sat) Generator 110 Condenser Evaporator 14 A prototype rig, sized to deliver 1.5kW of cooling was designed and manufactured at Industrial Design Consultancy Ltd. A jet-pump design, based on preliminary CFD, was injection moulded to reduce cost and allow for easy duplication. Once again, immersion heaters, controlled via a thermostat, were used to simulate the thermal input (3kW) available from 6m 2 of solar thermal panels during periods of peak solar irradiance. A water cooled condenser was employed to dissipate heat. Operational tests of the prototype yielded an experimental data-set, which was used to inform the theoretical performance graphed in figure 7. 5

6 Figure 7 Theoretical performance of AC prototype 1 Figure 8 Photo of AC prototype 2 This prototype provided COP cool s of up to 0.5 however it was felt that higher COP cool s could be achieved with revisions to the jet-pump design. Following a revised CFD study, a second prototype was designed and manufactured and a new jet pump installed. This second prototype employed an air-cooled condenser to better reflect a production application. A photo of the second prototype is shown in figure 8 above. Following commissioning of the second prototype, a stable state of operation was achieved and testing of the unit yielded the following experimental results. Table 3 AC prototype 2 experimental results Parameter P(sat) T(sat) T(on) T(off) Q (MPa) ( o C) ( o C) ( o C) (W) Evaporator (refrigerant) Evaporator (air-side) * Generator (refrigerant) <3000 Condenser (refrigerant) ~ 4500 Condenser (air-side) Critical Condenser Condition The cooling power of the system was evaluated by measuring the air-flow and temperature change across the evaporator. Immersion heaters were used to power a hot oil circuit. Generator heat input was not measured directly but calculated using the refrigerant mass flow (through the primary nozzle) and the enthalpy change across the generator (based upon generator temperature and pressure measurements). Table 4 AC prototype 2 performance calculations Parameter Key Value Units Formula Air flow m e kg s -1 m e = e A e c e Evaporator heat rate Qe W Q e = m e C p (T on T off ) Mass flow through nozzle m g kg s -1 Generator heat rate Q g 2844 W Q g = m g (h out h in ) Coefficient of performance COP cool COP cool = Q e /Q g Entrainment Ratio R m m e /m g 6

7 These calculations make the conservative assumption that the refrigerant entered the generator at the same temperature as the condensate coming from the condenser, (i.e. 26 o C), with enthalpy h in and at discharged from the generator with enthalpy h out (a dry-saturated vapour at 110 o C), 6. JET PUMP AIR CONDITIONER (AC) PROTOTYPE RESULTS DISCUSSION The data set provides a COP cool of which represents a significant achievement. From experimental results the critical condenser temperature, listed in table 3, was estimated to be just under 40 o C(sat). Using the experimental COP cool value the variations in COP cool with evaporator and generator temperatures have been estimated. These results are described in Figures 9 and Te = 12C Te = 15C Te = 10C Tg = 100C Tg = 95C 0.4 COPcool 0.3 Te = 8C COPcool 0.4 Tg = 110C Tg = 105C Critical condenser pressure at Tg = 110C condenser temperature condenser temperature Fig 9 COP cool /T c at T g =110 o C with variations of T e Fig 10 COP cool /T c at T e =15 o C with variations of T g In order to raise the COP cool value to 0.8 it would be necessary to install a primary nozzle with a smaller throat diameter and to operate the system at a lower design generator temperature, (approximately 90 o C). The effects of this would be to raise the COP but reduce the critical condenser temperature, The system is operating close to the critical condenser pressure and theoretical maximum efficiency for the aforementioned operating conditions. During subsequent tests, a COP cool of over 0.7 was achieved at a lower condenser pressure however the system could not maintain stability. It is believed that an automated control system would address this problem. These results show that the system has the potential to deliver impressive performance however further development is required to achieve design point operation. One potential method of increasing the COP would be the introduction of heat recovery methods (e.g. evaporator suction line heat exchanger). 7. COMBINED SOLAR HEATING AND AIR CONDITIONING Based upon the experimental results obtained during testing of both the jet-pump refrigerator and jet-pump AC system, a thermal powered combination system is proposed that would provide AC duty during hotter periods via the jet-pump cooling circuit and also be capable of delivering heating via the same evaporator during colder periods. The theoretical performance of the unit in cooling mode is described in table 5 below 7

8 Mode Table 5 Theoretical performance data for combined AC and heating Location Temperature Temperature Temperature Refrigerant Air On Air Off Power Units ( o C) ( o C) ( o C) (kw) Evaporator Cooling (Summer) Solar input = 500W m -2 Generator Condenser Heating (Spring/Autumn) Evaporator Solar input = 300W m -2 Generator Heating (Winter) Evaporator Solar Input 150W m -2 Generator The system would be rated to deliver 4.5kW of peak cooling, based on a COP cool of 0.6 and hence requiring 7.5kW thermal input. Assuming a panel efficiency of 0.5 and peak solar irradiance of 1kW m -2, 15m 2 of panels would be required. A simplified schematic of the system in cooling mode is shown in on the left side of figure 11 below. Figure 11 Schematic layout of proposed system in heating and cooling mode During colder periods, the same infrastructure could be used to provide space heating. Given the same collector area an estimated value of kW of heating could be supplied directly to the evaporator. The simplified schematic for the system in heating mode is shown in on the right side of figure 11 above. When operating at lower temperatures, panel efficiency will increase (assumed at 75% for winter conditions). 8. CONCLUSION The performance of both the jet-pump refrigerator and jet pump AC prototypes have demonstrated the viability of such low cost systems in meeting many of the end-user requirements. Although the jet-pump refrigerator provides a relatively low COP, it may have applications in off grid locations for duties such as vaccine storage. The jet-pump AC unit was shown to meet cooling loads in-line with domestic requirements. The technology also lends itself to modular construction allowing the system to be scaled up to meet larger cooling duties. Aside from new-build system construction, we have also identified the possibility of augmenting existing AC systems to improve efficiency and reduce costs. An illustrative schematic of an installed AC /heater combo system is shown in figure 12 below. 8

9 Figure 12 Illustrative schematic of installed combo system We envisage the system to be packaged as three distinct units. The input energy would be provided by a solar thermal panel array located on the roof. The jet pump power unit would also incorporate the vapour generator and condenser and would be located at the same height as the panels, on an external face of the building. Finally, the evaporator unit would provide the cooling duty and would be located remotely within the living/work space. Hot water heating and storage would be an additional function during periods when the evaporator is not in use. Based on the following assumptions, the primary quantitative benefit of the system is an energy saving equivalent to almost 2 tonnes of CO2 emissions per annum with significant further savings when space-heating and hot water loads are taken into account. Cooling Load Q e = 4.5kW Load Standard vapour compression AC COP = 2.0 Thermally powered jet pump AC COP cool = 0.6 CO2 emmisions for UK electricity usage W co2 = kg kwhr -1 (Carbon Trust, 2008) Annual Use t a = 800 hrs Various improvements to the system are being considered for the next development iteration including additional heat-recovery methods to improve COP. In addition, we are considering the use of PCM in order to extend the operating time of the unit outside daylight hours. Based upon the results described it is believed that a jet-pump cooling system can be developed, at a cost competitive to that of sorption technologies, that will provide a viable solution to the solar cooling challenge. 9

10 NOMENCLATURE P Pressure (Pascals) Subscripts h Enthalpy (kj kg -1 ) T sat temperature ( o C) a Annual Liquid Volume (litres) c Condenser t Time (mins) e Evaporator Solar irradiance (watt m -2 ) g Generator COP coefficient of performance sat Saturation W CO2 CO2 emmisions (kg kwhr -1 ) throat Jet Pump Nozzle Throat Q Thermal Power (watts) cool Thermal (e.g COP cool = Q e /Q g ) Conductance (watt/kelvin) R m Entrainment Ratio Mass (kg) C p Specific Heat Capacity (J kg -1 K -1 ) A Area (m 2 ) e Air Density (kg m -3 ) (across evaporator) c e Air Velocity (m s -1 ) (across evaporator) m e Air flow (kg s -1 ) (across evaporator) Ratio of specific heat values v Specific Volume (m 3 kg -1 ) m Mass flow (kg s -1 ) REFERENCES [1] Alexis G.K, Karayiannis E.K, 2004 A solar ejector cooling system using refrigerant R134a in the Athens area. Department of Mechanical Engineering, Technical Education Institute of Piraeus, Athens, Greece, [2] Carbon Trust, Resources-conversion factors, [Online] Available at [Accessed 29/11/2009 ] [3] Chunnanond K Aphornratana S, Ejectors: applications in refrigeration technology. Renewable and Sustainable Energy Reviews, 8, [4] Cogeneration Technologies, Solar Energy Systems. Solar HVAC Systems. [Online] Available from: [Accessed 11 July 2008]. [5] Eames I.W, 2006 Absorption Refrigeration and Jet Pumps. Presented to the Institute of Refrigeration, London, 2 November. [6] Eames I. W. et al, 1995 A theoretical and experimental study of a small-scale steam jet refrigerator. Department of Mechanical and Process Engineering, University of Sheffield, Sheffield, UK. [7] Kim D.S, Infante Ferreira C.A, 2008 Solar refrigeration options a state-of-the-art review. International Journal of Refrigeration, 31, [8] Marchio D, 2008, High efficiency and low environmental impact air-conditioning systems. Ines Paris Tech: Centre for Energy and Processes. [Online] Available from; [Accessed 20 July 2008]. [9] Selvaraju A, Mani A, Experimental investigation on R134a vapour ejector refrigeration system. International Journal of Refrigeration 29, [10] Seyboth K et al Recognising the potential for renewable energy heating and cooling. Energy Policy, 2008, 36. Available from: [Accessed 7 May 2008]. [11] Sun D, Comparative study of the performance of an ejector refrigeration cycle operating with various refrigerants. Energy Conversion & Management, 40,