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Application of combined heat-and-power and absorption cooling in a supermarket
Applied Energy 63 (1999) 169±190
www.elsevier.com/locate/apenergy
Application of combined heat-and-power and absorption cooling in a supermarket G.G. Maidment a,*, X. Zhao b, S.B. Riat c, G. Prosser d a
School of Engineering Systems and Design, South Bank University, 103 Borough Road, London, SE1 0AA, UK b Department of Environmental Engineering, Taiyuan University of Technology, Taiyuan, Shanxi, People's Republic of China, 030024 c School of the Built Environment, University of Nottingham, University Park, Nottingham, NG7 2RD, UK d Stal Refrigeration Ltd, Unit 5, The Grand Union Oce Park, Packet Boat Lane, Uxbridge, UB8 2GH, UK
Abstract In recent years, it has become standard practice to consider Combined Heat-and-Power (CHP) systems for commercial buildings. CHP schemes are used, because they are an ecient means of power generation. Unlike conventional power stations, they produce electricity locally and thus minimise the distribution losses, however, they also utilise the waste heat from the generation process. In applications where there is a combined heating and electricity requirement, a very ecient means of energy production is achieved compared to the conventional methods of providing heating and electricity. With new initiatives from the UK government on reduced energy-use, energy-ecient systems such as CHP have been considered for new applications. This paper summarises the results of an investigation into the viability of CHP systems in supermarkets. The viability of conventional CHP has been theoretically investigated using a mathematical model of a typical supermarket. This has demonstrated that a conventional CHP system may be practically applied. It has also been shown that compared to the traditional supermarket design, the proposed CHP system will use slightly less primary energy and the running costs will be signi®cantly reduced. An attractive payback period of approximately 4 years has been calculated. Despite these advantages a considerable quantity of heat is rejected to atmosphere with this system and this is because the con®guration utilises the heat mainly for space heating which is only required for part of the year. To increase the utilisation time, a novel CHP/absorption system has been investigated. This con®guration provides a continuous demand for the waste heat, which is used to drive an absorption chiller that refrigerates propylene glycol to ÿ10 C for cooling the chilled-food cabinets. The results show this concept to be theoretically practical. The system has also been shown to be extremely ecient, with primary energy savings of approximately 20%, when
* Corresponding author. Tel.: 44+-(0)171-928-8989; fax: +44-(0)171-815-7699. E-mail address: [email protected] (G.G. Maidment) 0306-2619/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0306-2619(99)00026-4
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compared to traditional supermarket designs and this would result in signi®cant revenue cost savings as well as environmental bene®ts. Based upon these savings a payback period for this system of approximately 5 years has been demonstrated. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Supermarkets; Absorption cooling; Combined heat and power; CHP; Food refrigeration
1. Introduction It is predicted that the average global temperature could rise by between 1.5 and 4.5 K in the next 100 years [1]. The principal cause of global warming is the emission of certain gases into the Earth's atmosphere. These produce the `greenhouse eect', which traps the sun's heat within the atmosphere and causes the global temperature to rise. The consequences of global climate-change are far reaching and include the ¯ooding of low lying islands and coastal areas, starvation and the spread of disease. To reduce the impact of global warming, the United Kingdom agreed to reduce greenhouse gas emissions by 20% by the year 2010. A major source of emission in the UK is from CO2, which results from the burning of fossil fuels in the production of electricity. Combined heat-and-power (CHP) schemes are seen as a major component in the strategy of the UK government to reduce CO2 emissions, such that they have set a target of 10 GW of installed CHP by the year 2010 [2]. The basic argument in favour of CHP is that it is possible to produce heat and power more eciently compared to the conventional methods of providing electricity, via the national supply grid, and heat, using a gas-®red boiler system. Conventional power stations generate electricity and reject the heat as waste. This wastage together with losses in the transmission of electricity results in low overall eciency and high energy costs. CHP schemes are an ecient means of generation as they produce electricity locally and thus minimise the distribution losses. However, they also allow the heat output from the generation plant to be used for space or process heating. In applications where there is a combined heating and electricity requirement, a very ecient means of energy usage is produced compared to the conventional methods of providing heating and electricity. This is detailed in Fig. 1(a) and (b), which show the primary energy consumed by a conventional scheme compared with a CHP scheme to satisfy the same heating and electrical demand. From this, it may be seen that the conventional system shown uses 41% more primary energy than the CHP scheme [3]. The retail food industry is a large user of energy, consuming as much as 0.33% of UK's total consumption [4]. In the typical supermarket, electricity is consumed by lighting, equipment and the food refrigeration system. Energy is also consumed by gas-®red boilers, which are used to provide hot water for space heating, kitchen, processing and toilet facilities. This use of energy in the conventional supermarket is shown in Fig. 2. Whilst CHP was reported over 100 years ago [3], its use in industrial applications in the UK is limited to large installations. However, in recent years, with the development
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Fig. 1. (a) Conventional heat and power application; (b) combined heat and power applications.
Fig. 2. Schematic showing the use of energy in the traditional supermarket.
of pre-packaged small-scale units with exergetic eciencies often in excess of 50% [5], CHP has been successfully applied in new applications. Previous work carried out by Maidment et al. [6] demonstrated the potential for a combined heating-and-cooling system to reduce the overall primary energy consumed by the supermarket. This used a gas engine to drive directly a refrigeration compressor and utilised the waste heat for store heating. Whilst energy savings were identi®ed with this system, it required high capital costs, as a separate engine was required to drive each compressor. However, it was suggested that this could be overcome if a conventional CHP system was considered rather than the direct drive engine. This paper describes an investigation into the viability of such a system in a supermarket installation. After a brief description of the typical supermarket, the electrical energy and heating requirements are de®ned. The method used to consider the viability of this system is then described and a number of con®gurations investigated. To fully utilise the bene®ts of heat and electricity cogeneration of CHP, and to further reduce the overall energy consumption of the supermarket, an alternative system that combines absorption cooling with CHP is then investigated. This system, shown in Fig. 3, uses the CHP unit to produce electricity to drive a low-temperature vapour-compression refrigeration cycle and operate store lighting, equipment and
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Fig. 3. Schematic showing the concept of the combined system.
the HVAC system. Heat produced by the CHP system is used to power an absorption chiller to provide mono-propylene glycol at ÿ10 C for cooling the chilled food cabinets. Heat is also used to satisfy the space-heating and hot-water demands. This paper also describes the investigation into the viability of this system. 2. The typical supermarket and energy consumption 2.1. The typical supermarket This investigation is based upon an existing Sainsbury's supermarket at Penge in London, which has recently been refurbished. The total area of the store is approximately 2000 m2, and there are two ¯oors. The ground ¯oor includes the retail area, cold stores, food preparation/processing areas, dry stores and a restaurant. The ®rst ¯oor area includes oces, a sta restaurant and a kitchen [7]. The store has been designed for approximately 700 people and has been assumed open from 7 am to 10 pm, 7 days a week. 2.2. Requirements of the conventional supermarket The heating, ventilation, refrigeration, lighting and general electrical requirements of the typical supermarket are outlined below. 2.2.1. Heating and ventilation design The traditional heating and ventilation system used in the supermarket consists of a mechanical ventilation system, which includes an air-handling unit complete with heater battery and fan. A gas-®red boiler provides hot water for the air-handling unit. Because the supermarket cabinets produce a large cooling eect, the supermarket requires heating for a large period of the year and for this reason mechanical air-cooling is not utilised. Despite this and the fact that the heating system runs only during trading hours (16 h/day), the supermarket is assumed to operate close to the design condition (20 C, 50% RH) throughout the year.
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The loads considered in the calculation of the overall heating-load include lighting, equipment and personnel load, solar gain, conduction through the walls, ventilation loads and the domestic hot-water load. A summary of the peak heat-loads calculated for the typical supermarket is shown in Table 1. The solar load was calculated using a method given by ASHRAE [8]. The conductive load was considered to be mainly steady state as the building was of lightweight construction. Internal and ventilation loads were calculated from data provided by the retailer. The domestic hot-water load was calculated using data from the CIBSE guide [9]. 2.2.2. Refrigeration system design In the supermarket, the food cabinets are grouped by product storage temperature into chilled and frozen food categories. A schematic of the refrigeration system is shown in Fig. 4. This shows that single-stage compressors are used for both duties, with individual suction lines but a common discharge, allowing for a single condenser system. Normally to minimise the risk of failure, there are two individual central systems, each satisfying 50% of the load. The compressors used in each system are constructed onto a base plate to form a compressor package, which also includes a liquid receiver and multi-station manifolds for individual liquid, suction and defrost gas connections. The compressors used are Bitzer screws and their operating conditions are shown in Table 2. The peak cabinet loads are also shown in Table 2 and it is assumed that cabinets operate continually through out the year at approximately 80% of the peak cooling-load [10]. The condensers are ®tted with head pressure control to ensure that the minimum condensing temperature is always maintained. 2.3. Calculation of the annual energy consumed by the conventional store The viability of the CHP system options was assessed by comparing the capital and revenue costs with those for the conventional format. Electricity consumption was calculated by considering the energy consumed by the lighting, equipment, the refrigeration system, cabinets, cold stores and the HVAC system. The gas consumption for the traditional system was calculated by considering the energy consumed by the boiler in satisfying the heat loads. Table 1 Heat load under design conditions for the supermarket store Heat load at design (kW)
% of total load
Fabric Air in®ltration Solar Hot water Lights Occupancy Heat absorption by cabinets
ÿ230.35 ÿ324.39 17.14 ÿ10.20 89.87 85.16 ÿ189.6
40.96 57.68 ÿ3.05 1.81 ÿ15.98 ÿ15.14 33.71
Total
ÿ562.37
100
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Fig. 4. Schematic of the arrangement of refrigeration system no. 1.
Table 2 Operation conditions and peak cooling-loads for LT and HT systems System type
Saturated suction temperature ( C)
Peak cooling load required (kW)
LT1 HT1 LT2 HT2
ÿ35 ÿ10 ÿ35 ÿ10
23 100 22 100
Design delivery temperature ( C)
Minimum delivery temperature ( C)
45
25
The method used to calculate the energy consumption for the conventional and CHP system designs was the BIN method [11]. This was used because it accurately calculates the annual electricity and heating energy consumption for a system operating under part-load conditions. It achieves this by making instantaneous energy calculations at dierent outdoor air temperatures and weighting each result by the number of hours that each temperature occurs throughout the year. Mean coincident wet-bulb data for each dry-bulb bin were used to calculate the latent coolingload due to in®ltration. The method of calculating the solar gain proposed by ASHRAE [8] was integrated into the BIN model. A summary of the design electrical loads and annual electricity consumption calculated for the typical supermarket is shown in Table 3. The total electricity consumption for the Penge supermarket was calculated to be 2,367,403 kWh/annum.
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The annual gas consumption calculated was 1,128,815 kWh. By using the average generation and distribution eciency for electricity delivered by the national grid for 1997 [12], the total primary-energy consumed by the conventional store was estimated to be approximately 7,892,821 kWh/annum. The total electrical and heating demand calculated at dierent ambient temperatures during daytime and night-time operation are shown in Figs. 5 and 6, respectively. It can be seen that the electrical load is fairly constant but rises slightly when the ambient temperature rises. This is due to a higher compressor-work requirement at higher ambient-temperatures and condenser pressures. It can also be seen that Table 3 Design electricity loads and annual electricity consumption for the supermarket store Categories Design electricity load (kW) Annual electricity consumption (kWh)
Lighting
Refrigeration
Cabinets and cold stores
HVAC
Micellaneous
Total
132
110
75
23
13
354
756,880
808,831
549,818
133,504
118,370
2,367,402
Fig. 5. Traditional day-time heat and electricity demand against ambient temperature.
Fig. 6. Traditional night-time heat and electricity demand against ambient temperature.
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during daytime operation, the thermal load reduces linearly with the air temperature. This is because the total heat gain is largely dependent on the fabric/in®ltration gain and the in¯uence of solar radiation is small. When the ambient temperature is above 16 C, heating within the store area is unnecessary due to the heat gain from lighting and occupants. 3. General application of CHP in Sainsbury's supermarket 3.1. General concept of CHP system applied in the supermarket The con®guration of the initial CHP system applied in the supermarket is detailed in Fig. 7. The CHP package comprised of three main components: an engine, a generator and heat exchangers. The engine shown is a combustion engine, which drives the generator, used to produce electricity for lighting, internal equipment, refrigeration equipment and the HVAC system. A parallel type generator has been used. This allows any de®cit in electricity to be made up from the national grid and surplus electricity to be exported. Heat exchangers recover the heat rejected during generation from the engine jacket and exhaust gas, providing water temperatures normally between 75 and 90 C for heating [13], although some systems may be con®gured to produce MTHW medium temperature hot-water up to 130 C [14]. A gas-®red boiler is also shown to supplement the heat provided by the CHP unit when necessary and a waste-heat exchanger is used to reject unwanted heat to the atmosphere. The energy inputs and outputs for a standard CHP package are shown in Table 4 [15]. Table 4 Energy distribution in a typical CHP unit Total energy input (%)
Electricity production (%)
Heat production (%)
Energy loss (%)
100
30
50 Including: Engine jacket: 28.6 Heat exchanger: 21.5
20 Including: Generator loss: 1.4 Engine surface radiation: 4.3 Latent heat in exhaust gas: 10 Sensible heat in exhaust gas: 4.3
3.2. Initial CHP investigations Using the BIN model, the economics of installing and operating speci®c CHP con®gurations were investigated. In each case, the annual energy consumption was calculated using a modi®ed BIN model and this was compared against that calculated for the traditional system. The equipment selected is described and the capital and maintenance costs de®ned. The viability of each case is considered using a pay-
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Fig. 7. Schematic showing CHP system in supermarket.
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back period calculation. Typical maintenance costs per unit output were used [13] and equipment manufacturers were consulted for capital costs. The cost of electricity used was £0.05/kWh for daytime operation and £0.027/kWh during `o-peak' operation [16]. The gas costs used were £0.0065/kWh for continuous usage and £0.009/kWh for a seasonal charging rate [17]. The salient results of these investigations are shown below. 3.3. Investigation 1Ðconventional CHP, heat-based system, 24-hour operation A heat-based system, that satis®ed the heat requirement of the supermarket, was investigated. The CHP unit was selected with a heat capacity of 470 kWt, which approached the peak heating load. With this system, a small de®cit between the peak heating load and the CHP output occurred for a few hours and, to oset this de®cit, a small gas-®red boiler was required. To satisfy the heat requirement of the store, the speed of the unit was modulated between 1000 and 2600 rpm as necessary. However, as the minimum heat output from the CHP unit was in excess of the minimum heat load, a heat exchanger was required to reject the waste heat. The thermal and electricity output from the CHP system was calculated and compared against the thermal and electricity demand of the supermarket as shown in Fig. 8. From Fig. 8(a), it can be seen that during the day time the CHP unit modulates its heat output to generally satisfy the heat demand, although some rejected heat is required to balance the demand at the higher ambient temperatures. From Fig. 8(b), it can be seen that during night-time operation, all of the heat generated by the CHP unit is rejected. This is because the typical supermarket is not heated during non-trading hours. From Fig. 8(c), it can be seen that during both day and night time, electricity produced by the CHP does not satisfy the total demand and additional electricity is required from the national grid. However, under lower ambient temperatures at night, some electricity is exported to the grid. The comparative energy consumption and costs calculated for this system are shown in Table 5, together with the additional maintenance and capital costs
Table 5 Summary of calculation results for CHP system Ð option 1 Additional gas consumption by combined unit
Saving in electricity consumption by combined unit
Saving of Additional gas Cost of electricity additional consumed (kWh/annum) gas (£/annum) (kWh/annum) Day time 2,513,032
13,513
Night time
Cost saving Ð electricity (£/annum) Day time
Maintenance Annual revenue Additional Payback capital cost period cost saving cost of combined (years) (£/annum) (£/annum) unit (£)
Night time
544,831 550,795 27,242 14,871 7,383
21,217
190,510
8.98
Fig. 8. Output/demand pro®les for `heat based' CHP systemÐoption 1.
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required. This has been used to calculate a payback period of 9 years and a primary energy saving of 8% compared with the conventional system. 3.4. Investigation 2Ðconventional CHP, `electricity based' system, 24-hour operation This con®guration ensured that the electricity requirement of the store was always satis®ed and the CHP unit was selected to meet the peak electricity-load. This unit had a peak electrical capacity of 290 kWe and heat output of 490 kWt. As this system was con®gured to always satisfy the electrical demand, a small additional boiler was required to supplement the thermal output when required and a waste-heat exchanger was necessary to reject heat under low thermal-demand conditions. The thermal and electricity output for the CHP system and corresponding energy demand calculated for this option are shown in Fig. 9. From this it can be seen that the CHP unit always satis®es the electrical demand of the supermarket. However, there are considerable periods when the thermal output from the CHP system signi®cantly exceeds the heat demand. This occurs at night and when the ambient temperature is above ÿ3 C during trading hours. The comparative energy consumption and costs calculated for this option are shown in Table 6. This shows a payback period of 4.21 years for this schedule, which is a signi®cant improvement on the heat-based system. However, due to the large amount of heat wasted by this system, primary-energy savings do not occur and the energy usage is similar to that for the traditional system. 3.5. Investigation 3Ðconventional CHP, electricity based system, daytime operation The previous option showed that the heat generated at night was not utilised. Due to this and because of the very low cost of `o-peak' electricity supplied by the grid, it was questionable whether generation using the CHP unit during night-time is viable. The economics of using the CHP unit during daytime only was investigated. The CHP con®guration investigated was identical to that detailed in Section 3.2, although the unit did not operate during night-time. The daytime thermal and electrical CHP output and store demand load pro®les are therefore similar to those shown in Fig. 9(a) and (c). The overall energy consumed and the capital cost for this option compared to the traditional system are shown in Table 7. This shows a payback period of 4.1 years, which is a slight improvement on the continuous operation option. Also, a small saving in primary-energy consumption is produced and this con®rms that operation during night-time is not recommended. 4. Application of a combined CHP and absorption cooling system Section 3 showed that although a reasonable payback period may be achieved when a conventional CHP system is considered for a supermarket, a considerable amount of heat was wasted because there was only a heat demand for a small proportion of the year.
Fig. 9. Output/demand pro®les for `electricity-based' CHP systemÐoption 2 (24 h operation).
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Table 6 Summary of calculation results for CHP system Ð option 2 Additional gas consumption by combined unit
Saving in electricity consumption by combined unit
Saving of Cost of Additional electricity gas consumed additional (kWh/annum) gas (£/annum) (kWh/annum) Day time 6,822,631
41,525
Night time
Maintenance Annual revenue Additional capital Payback cost cost saving cost of period (£/annum) (£/annum) combined unit (£) (years)
Cost saving Ð electricity (£/annum) Day time
Night time
1816607 550,795 90,830 14,871 16,098
48,079
202,510
4.21
Table 7 Summary of calculation results for CHP system Ð option 3 Additional gas consumption by combined unit
Saving in electricity consumption by combined unit
Additional Cost of Saving of gas consumed additional electricity (kWh/annum) gas (£/annum) (kWh/annum) Day time 4,992,109
29,441
Cost saving Ð electricity (£/annum)
Night Day time time
1,816,607 0
Maintenance Annual revenue Additional capital Payback cost cost saving cost of period (£/annum) (£/annum) combined unit (£) (years)
90,830
Night time 0
11,788
49,601
202,510
4.08
Prosser and Maidment [18] reported that this could be overcome if heat from the CHP unit is used to power an absorption chiller, which provides ÿ10 C glycol for cooling of the chilled-food cabinets. This novel con®guration provides a demand for the hot-water source, which allows the CHP unit to be fully utilised even in the summer months. An investigation to understand the feasibility and viability of this novel system was carried out and the results of this investigation are detailed in this section. 4.1. General description of the novel system A schematic showing the proposed system is detailed in Fig. 10. The CHP unit has been selected to satisfy the electricity demand of the Penge supermarket. This is used for lighting, equipment, the HVAC system and a vapour compression refrigeration system serving the frozen food cabinets only. The system is also con®gured to allow heat produced by the CHP unit to be used for space heating, to provide hot water and to drive an absorption chiller to cool glycol for circulation to chilled food cabinets. The glycol temperatures assumed to satisfy the chilled cabinets were ÿ10 C ¯ow and ÿ6 C return [19], and the performance data used for this system are shown in the Appendix.
Fig. 10. Schematic showing of electricity-based CHP and absorption HT refrigeration system.
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Fig. 11. Relationship between COP of the chiller and BIN temperatures.
The absorption chiller selected to achieve these glycol temperatures was a bespoke single-stage ammonia/water chiller. The performance of the absorption chiller was obtained using manufacturer's data [20] and is shown in Fig. 11. To produce the required glycol temperatures, it was necessary to power the absorption chiller using medium-temperature hot-water (MTHW) at 124 C, which required the use of a high-temperature CHP engine. 4.2. Investigation 4Ðinitial analysis of the combined CHP/absorption system The viability of the system described above was investigated with a CHP unit with the following capacities at the design conditions [14]
Electricity output: Heat recovered into primary water circuit: Heat recovered from cool exhaust gas: Engine's gas-consumption:
251 293 68 711
kW kW kW kW
The thermal and electricity output for this con®guration and the corresponding energy demand was simulated using the computer model and the results from this are shown in Fig. 12. From this, it can be seen that the electricity demand is always satis®ed by the CHP output. Also, it can be seen that the heat supplied by the CHP Table 8 Summary of calculation results for CHP system Ð option 4 Additional gas consumption by combined unit
Saving in electricity consumption by combined unit
Saving of Cost of Additional electricity gas consumed additional (kWh/annum) gas (£/annum) (kWh/annum) Day time 5,294,105
31,590
Night time
Maintenance Annual revenue Additional Payback cost cost saving capital cost of period (£/annum) (£/annum) combined unit (£) (years)
Cost saving Ð electricity (£/annum) Day time
Night time
1,816,607 550,795 90,830 14,871 12,130
61,982
373,994
6
Fig. 12. Output/demand pro®les for `electricity-based' CHP/absorption systemÐoption 4.
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unit is fully utilised, although supplementary heat provided by the gas ®red boiler is always required. The economics of this option was investigated using a revised BIN model and the comparative energy consumption and costs for this scheme compared to the traditional format are shown in Table 8. This shows a payback period of 6 years is produced for this schedule and the primary energy consumed is 20% less than the traditional system. 4.3. Investigation 5Ðapplication of absorption chillers for frozen cabinets The use of an absorption chiller for providing glycol to satisfy the frozen food cabinets has been considered and was found not to be practical. Whilst absorption chillers are available which can operate at temperatures down to ÿ60 C [20], these require unacceptably-high water-temperatures which are not available with current small-scale CHP systems. For this reason, this option has not been considered further. 4.4. Investigation 6Ðfrozen food cabinets with integral cascade condensers The feasibility of using chilled propylene glycol as a heat source for frozen-food cabinets employing integral refridgeration units with cascade condensers has been investigated and the results indicate that the advantage of this is small. Estimates indicate that the payback period would be similar or slightly higher than that calculated for the systems analysed in Section 4.2. Furthermore, the cascade system would only operate when there is a chilled glycol supply. During maintenance peri-
Fig. 13. Relation between cooling duty and relative price of absorption chillers.
Table 9 Summary of payback periods and primary energy consumption for all options Options
Traditional system
1
2
3
4
Payback period (years) Primary-energy consumption (kWh/annum)
± 7,892,821
8.98 7,275,492
4.21 7,951,445
4.08 7,674,992
6 6,422,900
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ods, the frozen-food system would not be able to operate without the chilled food system running. For this reason, this option is not recommended. 5. Conclusions The investigation has demonstrated that a conventional CHP system may be practically applied to provide heating and generate electricity in the supermarket. A number of options of conventional CHP have been investigated theoretically using a computer model of a small, Sainsbury's supermarket. This has shown that small primary energy savings are achievable and attractive payback periods are possible. Using an `electricity-based' con®guration, operating during trading hours only, saves approximately 3% of primary-energy compared to the conventional heat-andpower systems. For this system, a payback period of approximately 4.1 years has been calculated. Despite these advantages, a considerable quantity of heat is rejected to atmosphere with this system because this con®guration utilises the heat mostly for space heating, which is only required for part of the year. To increase the utilisation period, a novel CHP/absorption system has been investigated. This system uses a CHP unit to generate electricity for lighting, equipment and for a vapour-compression refrigeration system serving frozen food cabinets. As well as using the waste heat for space heating and to provide hot water, this system utilises the heat to drive an absorption chiller, which produces propylene glycol at ÿ10 C for cooling the chilled-food cabinets. This con®guration provides a continuous demand for the waste heat. This investigation has shown that this system is practical. However, in order to achieve the desired glycol temperatures, a CHP system which generates mediumtemperature hot-water is required. The investigation has shown this con®guration to be extremely ecient, as primary energy savings of 20% are achievable, as shown in Table 9. When taking into consideration the additional capital cost of this con®guration, a payback period of 6 years has been calculated. The main reason for this payback being slightly higher than that calculated for the conventional CHP system is the large additional capital cost of the absorption chiller. This is partly because the chiller selected is one of the smallest models in the manufacturer's range; this size of chiller was required because the typical store modelled was one of the smaller Sainsbury stores. For larger stores, better payback periods of between 4 and 5 years may be achieved as the unit cost of the absorption chiller reduces signi®cantly as the size increases as shown in Fig. 13. Also this equipment is currently designed and constructed on a bespoke basis, with increased demand and economies of scale further reductions in payback period would be realised. This investigation has subsequently been extended to consider the viability of incorporating an absorption/ secondary refrigerant system into the CHP scheme to provide the cooling for frozen-food cabinets. However, this con®guration has been shown to be impractical because absorption chillers capable of operating at frozen food temperatures require extremely high water-temperatures, which cannot be achieved with commercial CHP systems currently available.
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Acknowledgements The authors greatly appreciate the support of Sainsbury's Supermarkets Ltd, for this work and allowing the results to be published. The technical input of Dr Robert Tozer of Waterman Gore Ltd, Mr. J. Knowles of Air CHP Ltd, Peterborough and Mr. D.W. Maidment of ChemEng Ltd, Dartford is also valued. Appendix Table A1 Speci®cation for traditional vapour compression installation HT system
LT system
System description
Single-stage compressors with economisers, air-cooled condensers and direct expansion evaporators
Single-stage compressors with economisers, air-cooled condensers and direct expansion evaporators
Refrigerant Design condensing temperature Design evaporating temperature Design saturated suction temperature Design suction temperature Design COP Fan motor size condensers
R404a 45 C ÿ10 C ÿ12 C +10 C 3 6.2 kW
R404a 45 C ÿ33 C ÿ36 C ÿ10 C 1.85 1.6 kW
Table A2 Speci®cation for installation utilising CHP and absorption HT system System description
Absorption refrigerant Secondary re®rgerant Glycol-¯ow temperature Glycol-return temperature
Central heat driven absorption chiller connected to dry coolers and providing chilled glycol for circulation to the chillfood system Ammonia/water Propylene glycol ÿ10 C ÿ6 C (continued on next page)
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Table A2 (continued) Design dry-cooler ¯ow temperature Design dry-cooler return temperature Design COP Fan motor size of dry cooler Dry cooler water-pump motor size Glycol-pump motor size LT system System description
Refrigerant Design condensing-temperature Design evaporating-temperature Design saturated suction-temperature Design suction-temperature Design COP Fan motor-size condenser
30.5 C 33.5 C 0.51 6.6 kW 5.76 kW 4.5 kW Single-stage compressors with economisers, air-cooled condensers and direct expansion evaporators R404a 45 C ÿ33 C ÿ36 C ÿ10 C 1.85 1.6 kW
References [1] Anon. The environment in your pocket: key facts and ®gures on the Environment of the United Kingdom. DETR, 1998. [2] Reay DA, Heppenstall T. The Potential for small-scale CHP in industrial sectorsÐa scoping study, August 1997. [3] Eastop TD, Croft DR. Energy eciency for engineers and technologists. 1st ed. Harlow, UK: Longman Scienti®c and Technical, 1990 p. 335. [4] Anon. The energy report, change and opportunity. DTI, June 1996, pp. 169±79. [5] Bejan A, Tsatsaronis G, Moran, M. Thermal design and optimization. John Wiley and Sons, 1996. [6] Maidment GG, Zhao X, Riat SB. Investigation into the viability of a combined heating and cooling system using a gas engine driven compressor in a supermarket. CIBSE National Conference, Harrogate, 1999. [7] Longcross. Personal Communication, Design of Sainsbury's supermarkets Ltd Ð Penge, May, 1998. [8] ASHRAE. Fundamentals Handbook. Atlanta, USA: ASHRAE, 1993. pp. 28.13±28.15 [9] Anon. CIBSE guide, vol. A. London: The Chartered Institution of Building Services Engineers, 1986. [10] ETSU. Night blinds on refrigerated cabinets. Good practice case study 223, May 1995. [11] ASHRAE. Fundamentals handbook. ASHRAE, Atlanta, USA, pp. 28.9±28.12, 1989. [12] Petrie L, editor. Digest of United Kingdom Energy Statistics. London: The Government Statistical Service and The Stationary Oce for the DTI, 1998. [13] ETSU. Introduction to small-scale combined heat and power. Good practice guide 3, pp. 3±19, March 1995. [14] Knowles J. Air CHP Ltd, Peterborough, Personal Communication, 23 September, 1998. [15] Knowles J. Air CHP Ltd, Peterborough, Personal Communication, 10 September, 1998. [16] London Electricity. Electricity prices for your business. London Electricity Plc, April 1998. [17] Butlin C. Mobil Gas Marketing (UK) Ltd., Personal Communication, 10 February 1998.
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[18] Prosser GT, Maidment GG. Investigation into the viability of CHP in cold-storage facilities. International Institute of Refrigeration Conference, Refrigerated Transport, Storage and Display Conference, 29 March 1998. [19] Maidment GG. Supermarket refrigeration with ammonia, the environmentally-friendly refrigerant. BEng(Hons) dissertation, South Bank University, London, 1991. [20] Colibri bv. http://www.colibri-stork.nl/, Colibri bv, Tentstraat 5a, 6291 Bc Vaals, The Netherlands.
www.elsevier.com/locate/apenergy
Application of combined heat-and-power and absorption cooling in a supermarket G.G. Maidment a,*, X. Zhao b, S.B. Riat c, G. Prosser d a
School of Engineering Systems and Design, South Bank University, 103 Borough Road, London, SE1 0AA, UK b Department of Environmental Engineering, Taiyuan University of Technology, Taiyuan, Shanxi, People's Republic of China, 030024 c School of the Built Environment, University of Nottingham, University Park, Nottingham, NG7 2RD, UK d Stal Refrigeration Ltd, Unit 5, The Grand Union Oce Park, Packet Boat Lane, Uxbridge, UB8 2GH, UK
Abstract In recent years, it has become standard practice to consider Combined Heat-and-Power (CHP) systems for commercial buildings. CHP schemes are used, because they are an ecient means of power generation. Unlike conventional power stations, they produce electricity locally and thus minimise the distribution losses, however, they also utilise the waste heat from the generation process. In applications where there is a combined heating and electricity requirement, a very ecient means of energy production is achieved compared to the conventional methods of providing heating and electricity. With new initiatives from the UK government on reduced energy-use, energy-ecient systems such as CHP have been considered for new applications. This paper summarises the results of an investigation into the viability of CHP systems in supermarkets. The viability of conventional CHP has been theoretically investigated using a mathematical model of a typical supermarket. This has demonstrated that a conventional CHP system may be practically applied. It has also been shown that compared to the traditional supermarket design, the proposed CHP system will use slightly less primary energy and the running costs will be signi®cantly reduced. An attractive payback period of approximately 4 years has been calculated. Despite these advantages a considerable quantity of heat is rejected to atmosphere with this system and this is because the con®guration utilises the heat mainly for space heating which is only required for part of the year. To increase the utilisation time, a novel CHP/absorption system has been investigated. This con®guration provides a continuous demand for the waste heat, which is used to drive an absorption chiller that refrigerates propylene glycol to ÿ10 C for cooling the chilled-food cabinets. The results show this concept to be theoretically practical. The system has also been shown to be extremely ecient, with primary energy savings of approximately 20%, when
* Corresponding author. Tel.: 44+-(0)171-928-8989; fax: +44-(0)171-815-7699. E-mail address: [email protected] (G.G. Maidment) 0306-2619/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0306-2619(99)00026-4
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compared to traditional supermarket designs and this would result in signi®cant revenue cost savings as well as environmental bene®ts. Based upon these savings a payback period for this system of approximately 5 years has been demonstrated. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Supermarkets; Absorption cooling; Combined heat and power; CHP; Food refrigeration
1. Introduction It is predicted that the average global temperature could rise by between 1.5 and 4.5 K in the next 100 years [1]. The principal cause of global warming is the emission of certain gases into the Earth's atmosphere. These produce the `greenhouse eect', which traps the sun's heat within the atmosphere and causes the global temperature to rise. The consequences of global climate-change are far reaching and include the ¯ooding of low lying islands and coastal areas, starvation and the spread of disease. To reduce the impact of global warming, the United Kingdom agreed to reduce greenhouse gas emissions by 20% by the year 2010. A major source of emission in the UK is from CO2, which results from the burning of fossil fuels in the production of electricity. Combined heat-and-power (CHP) schemes are seen as a major component in the strategy of the UK government to reduce CO2 emissions, such that they have set a target of 10 GW of installed CHP by the year 2010 [2]. The basic argument in favour of CHP is that it is possible to produce heat and power more eciently compared to the conventional methods of providing electricity, via the national supply grid, and heat, using a gas-®red boiler system. Conventional power stations generate electricity and reject the heat as waste. This wastage together with losses in the transmission of electricity results in low overall eciency and high energy costs. CHP schemes are an ecient means of generation as they produce electricity locally and thus minimise the distribution losses. However, they also allow the heat output from the generation plant to be used for space or process heating. In applications where there is a combined heating and electricity requirement, a very ecient means of energy usage is produced compared to the conventional methods of providing heating and electricity. This is detailed in Fig. 1(a) and (b), which show the primary energy consumed by a conventional scheme compared with a CHP scheme to satisfy the same heating and electrical demand. From this, it may be seen that the conventional system shown uses 41% more primary energy than the CHP scheme [3]. The retail food industry is a large user of energy, consuming as much as 0.33% of UK's total consumption [4]. In the typical supermarket, electricity is consumed by lighting, equipment and the food refrigeration system. Energy is also consumed by gas-®red boilers, which are used to provide hot water for space heating, kitchen, processing and toilet facilities. This use of energy in the conventional supermarket is shown in Fig. 2. Whilst CHP was reported over 100 years ago [3], its use in industrial applications in the UK is limited to large installations. However, in recent years, with the development
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171
Fig. 1. (a) Conventional heat and power application; (b) combined heat and power applications.
Fig. 2. Schematic showing the use of energy in the traditional supermarket.
of pre-packaged small-scale units with exergetic eciencies often in excess of 50% [5], CHP has been successfully applied in new applications. Previous work carried out by Maidment et al. [6] demonstrated the potential for a combined heating-and-cooling system to reduce the overall primary energy consumed by the supermarket. This used a gas engine to drive directly a refrigeration compressor and utilised the waste heat for store heating. Whilst energy savings were identi®ed with this system, it required high capital costs, as a separate engine was required to drive each compressor. However, it was suggested that this could be overcome if a conventional CHP system was considered rather than the direct drive engine. This paper describes an investigation into the viability of such a system in a supermarket installation. After a brief description of the typical supermarket, the electrical energy and heating requirements are de®ned. The method used to consider the viability of this system is then described and a number of con®gurations investigated. To fully utilise the bene®ts of heat and electricity cogeneration of CHP, and to further reduce the overall energy consumption of the supermarket, an alternative system that combines absorption cooling with CHP is then investigated. This system, shown in Fig. 3, uses the CHP unit to produce electricity to drive a low-temperature vapour-compression refrigeration cycle and operate store lighting, equipment and
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Fig. 3. Schematic showing the concept of the combined system.
the HVAC system. Heat produced by the CHP system is used to power an absorption chiller to provide mono-propylene glycol at ÿ10 C for cooling the chilled food cabinets. Heat is also used to satisfy the space-heating and hot-water demands. This paper also describes the investigation into the viability of this system. 2. The typical supermarket and energy consumption 2.1. The typical supermarket This investigation is based upon an existing Sainsbury's supermarket at Penge in London, which has recently been refurbished. The total area of the store is approximately 2000 m2, and there are two ¯oors. The ground ¯oor includes the retail area, cold stores, food preparation/processing areas, dry stores and a restaurant. The ®rst ¯oor area includes oces, a sta restaurant and a kitchen [7]. The store has been designed for approximately 700 people and has been assumed open from 7 am to 10 pm, 7 days a week. 2.2. Requirements of the conventional supermarket The heating, ventilation, refrigeration, lighting and general electrical requirements of the typical supermarket are outlined below. 2.2.1. Heating and ventilation design The traditional heating and ventilation system used in the supermarket consists of a mechanical ventilation system, which includes an air-handling unit complete with heater battery and fan. A gas-®red boiler provides hot water for the air-handling unit. Because the supermarket cabinets produce a large cooling eect, the supermarket requires heating for a large period of the year and for this reason mechanical air-cooling is not utilised. Despite this and the fact that the heating system runs only during trading hours (16 h/day), the supermarket is assumed to operate close to the design condition (20 C, 50% RH) throughout the year.
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173
The loads considered in the calculation of the overall heating-load include lighting, equipment and personnel load, solar gain, conduction through the walls, ventilation loads and the domestic hot-water load. A summary of the peak heat-loads calculated for the typical supermarket is shown in Table 1. The solar load was calculated using a method given by ASHRAE [8]. The conductive load was considered to be mainly steady state as the building was of lightweight construction. Internal and ventilation loads were calculated from data provided by the retailer. The domestic hot-water load was calculated using data from the CIBSE guide [9]. 2.2.2. Refrigeration system design In the supermarket, the food cabinets are grouped by product storage temperature into chilled and frozen food categories. A schematic of the refrigeration system is shown in Fig. 4. This shows that single-stage compressors are used for both duties, with individual suction lines but a common discharge, allowing for a single condenser system. Normally to minimise the risk of failure, there are two individual central systems, each satisfying 50% of the load. The compressors used in each system are constructed onto a base plate to form a compressor package, which also includes a liquid receiver and multi-station manifolds for individual liquid, suction and defrost gas connections. The compressors used are Bitzer screws and their operating conditions are shown in Table 2. The peak cabinet loads are also shown in Table 2 and it is assumed that cabinets operate continually through out the year at approximately 80% of the peak cooling-load [10]. The condensers are ®tted with head pressure control to ensure that the minimum condensing temperature is always maintained. 2.3. Calculation of the annual energy consumed by the conventional store The viability of the CHP system options was assessed by comparing the capital and revenue costs with those for the conventional format. Electricity consumption was calculated by considering the energy consumed by the lighting, equipment, the refrigeration system, cabinets, cold stores and the HVAC system. The gas consumption for the traditional system was calculated by considering the energy consumed by the boiler in satisfying the heat loads. Table 1 Heat load under design conditions for the supermarket store Heat load at design (kW)
% of total load
Fabric Air in®ltration Solar Hot water Lights Occupancy Heat absorption by cabinets
ÿ230.35 ÿ324.39 17.14 ÿ10.20 89.87 85.16 ÿ189.6
40.96 57.68 ÿ3.05 1.81 ÿ15.98 ÿ15.14 33.71
Total
ÿ562.37
100
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Fig. 4. Schematic of the arrangement of refrigeration system no. 1.
Table 2 Operation conditions and peak cooling-loads for LT and HT systems System type
Saturated suction temperature ( C)
Peak cooling load required (kW)
LT1 HT1 LT2 HT2
ÿ35 ÿ10 ÿ35 ÿ10
23 100 22 100
Design delivery temperature ( C)
Minimum delivery temperature ( C)
45
25
The method used to calculate the energy consumption for the conventional and CHP system designs was the BIN method [11]. This was used because it accurately calculates the annual electricity and heating energy consumption for a system operating under part-load conditions. It achieves this by making instantaneous energy calculations at dierent outdoor air temperatures and weighting each result by the number of hours that each temperature occurs throughout the year. Mean coincident wet-bulb data for each dry-bulb bin were used to calculate the latent coolingload due to in®ltration. The method of calculating the solar gain proposed by ASHRAE [8] was integrated into the BIN model. A summary of the design electrical loads and annual electricity consumption calculated for the typical supermarket is shown in Table 3. The total electricity consumption for the Penge supermarket was calculated to be 2,367,403 kWh/annum.
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The annual gas consumption calculated was 1,128,815 kWh. By using the average generation and distribution eciency for electricity delivered by the national grid for 1997 [12], the total primary-energy consumed by the conventional store was estimated to be approximately 7,892,821 kWh/annum. The total electrical and heating demand calculated at dierent ambient temperatures during daytime and night-time operation are shown in Figs. 5 and 6, respectively. It can be seen that the electrical load is fairly constant but rises slightly when the ambient temperature rises. This is due to a higher compressor-work requirement at higher ambient-temperatures and condenser pressures. It can also be seen that Table 3 Design electricity loads and annual electricity consumption for the supermarket store Categories Design electricity load (kW) Annual electricity consumption (kWh)
Lighting
Refrigeration
Cabinets and cold stores
HVAC
Micellaneous
Total
132
110
75
23
13
354
756,880
808,831
549,818
133,504
118,370
2,367,402
Fig. 5. Traditional day-time heat and electricity demand against ambient temperature.
Fig. 6. Traditional night-time heat and electricity demand against ambient temperature.
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during daytime operation, the thermal load reduces linearly with the air temperature. This is because the total heat gain is largely dependent on the fabric/in®ltration gain and the in¯uence of solar radiation is small. When the ambient temperature is above 16 C, heating within the store area is unnecessary due to the heat gain from lighting and occupants. 3. General application of CHP in Sainsbury's supermarket 3.1. General concept of CHP system applied in the supermarket The con®guration of the initial CHP system applied in the supermarket is detailed in Fig. 7. The CHP package comprised of three main components: an engine, a generator and heat exchangers. The engine shown is a combustion engine, which drives the generator, used to produce electricity for lighting, internal equipment, refrigeration equipment and the HVAC system. A parallel type generator has been used. This allows any de®cit in electricity to be made up from the national grid and surplus electricity to be exported. Heat exchangers recover the heat rejected during generation from the engine jacket and exhaust gas, providing water temperatures normally between 75 and 90 C for heating [13], although some systems may be con®gured to produce MTHW medium temperature hot-water up to 130 C [14]. A gas-®red boiler is also shown to supplement the heat provided by the CHP unit when necessary and a waste-heat exchanger is used to reject unwanted heat to the atmosphere. The energy inputs and outputs for a standard CHP package are shown in Table 4 [15]. Table 4 Energy distribution in a typical CHP unit Total energy input (%)
Electricity production (%)
Heat production (%)
Energy loss (%)
100
30
50 Including: Engine jacket: 28.6 Heat exchanger: 21.5
20 Including: Generator loss: 1.4 Engine surface radiation: 4.3 Latent heat in exhaust gas: 10 Sensible heat in exhaust gas: 4.3
3.2. Initial CHP investigations Using the BIN model, the economics of installing and operating speci®c CHP con®gurations were investigated. In each case, the annual energy consumption was calculated using a modi®ed BIN model and this was compared against that calculated for the traditional system. The equipment selected is described and the capital and maintenance costs de®ned. The viability of each case is considered using a pay-
177
Fig. 7. Schematic showing CHP system in supermarket.
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back period calculation. Typical maintenance costs per unit output were used [13] and equipment manufacturers were consulted for capital costs. The cost of electricity used was £0.05/kWh for daytime operation and £0.027/kWh during `o-peak' operation [16]. The gas costs used were £0.0065/kWh for continuous usage and £0.009/kWh for a seasonal charging rate [17]. The salient results of these investigations are shown below. 3.3. Investigation 1Ðconventional CHP, heat-based system, 24-hour operation A heat-based system, that satis®ed the heat requirement of the supermarket, was investigated. The CHP unit was selected with a heat capacity of 470 kWt, which approached the peak heating load. With this system, a small de®cit between the peak heating load and the CHP output occurred for a few hours and, to oset this de®cit, a small gas-®red boiler was required. To satisfy the heat requirement of the store, the speed of the unit was modulated between 1000 and 2600 rpm as necessary. However, as the minimum heat output from the CHP unit was in excess of the minimum heat load, a heat exchanger was required to reject the waste heat. The thermal and electricity output from the CHP system was calculated and compared against the thermal and electricity demand of the supermarket as shown in Fig. 8. From Fig. 8(a), it can be seen that during the day time the CHP unit modulates its heat output to generally satisfy the heat demand, although some rejected heat is required to balance the demand at the higher ambient temperatures. From Fig. 8(b), it can be seen that during night-time operation, all of the heat generated by the CHP unit is rejected. This is because the typical supermarket is not heated during non-trading hours. From Fig. 8(c), it can be seen that during both day and night time, electricity produced by the CHP does not satisfy the total demand and additional electricity is required from the national grid. However, under lower ambient temperatures at night, some electricity is exported to the grid. The comparative energy consumption and costs calculated for this system are shown in Table 5, together with the additional maintenance and capital costs
Table 5 Summary of calculation results for CHP system Ð option 1 Additional gas consumption by combined unit
Saving in electricity consumption by combined unit
Saving of Additional gas Cost of electricity additional consumed (kWh/annum) gas (£/annum) (kWh/annum) Day time 2,513,032
13,513
Night time
Cost saving Ð electricity (£/annum) Day time
Maintenance Annual revenue Additional Payback capital cost period cost saving cost of combined (years) (£/annum) (£/annum) unit (£)
Night time
544,831 550,795 27,242 14,871 7,383
21,217
190,510
8.98
Fig. 8. Output/demand pro®les for `heat based' CHP systemÐoption 1.
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required. This has been used to calculate a payback period of 9 years and a primary energy saving of 8% compared with the conventional system. 3.4. Investigation 2Ðconventional CHP, `electricity based' system, 24-hour operation This con®guration ensured that the electricity requirement of the store was always satis®ed and the CHP unit was selected to meet the peak electricity-load. This unit had a peak electrical capacity of 290 kWe and heat output of 490 kWt. As this system was con®gured to always satisfy the electrical demand, a small additional boiler was required to supplement the thermal output when required and a waste-heat exchanger was necessary to reject heat under low thermal-demand conditions. The thermal and electricity output for the CHP system and corresponding energy demand calculated for this option are shown in Fig. 9. From this it can be seen that the CHP unit always satis®es the electrical demand of the supermarket. However, there are considerable periods when the thermal output from the CHP system signi®cantly exceeds the heat demand. This occurs at night and when the ambient temperature is above ÿ3 C during trading hours. The comparative energy consumption and costs calculated for this option are shown in Table 6. This shows a payback period of 4.21 years for this schedule, which is a signi®cant improvement on the heat-based system. However, due to the large amount of heat wasted by this system, primary-energy savings do not occur and the energy usage is similar to that for the traditional system. 3.5. Investigation 3Ðconventional CHP, electricity based system, daytime operation The previous option showed that the heat generated at night was not utilised. Due to this and because of the very low cost of `o-peak' electricity supplied by the grid, it was questionable whether generation using the CHP unit during night-time is viable. The economics of using the CHP unit during daytime only was investigated. The CHP con®guration investigated was identical to that detailed in Section 3.2, although the unit did not operate during night-time. The daytime thermal and electrical CHP output and store demand load pro®les are therefore similar to those shown in Fig. 9(a) and (c). The overall energy consumed and the capital cost for this option compared to the traditional system are shown in Table 7. This shows a payback period of 4.1 years, which is a slight improvement on the continuous operation option. Also, a small saving in primary-energy consumption is produced and this con®rms that operation during night-time is not recommended. 4. Application of a combined CHP and absorption cooling system Section 3 showed that although a reasonable payback period may be achieved when a conventional CHP system is considered for a supermarket, a considerable amount of heat was wasted because there was only a heat demand for a small proportion of the year.
Fig. 9. Output/demand pro®les for `electricity-based' CHP systemÐoption 2 (24 h operation).
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Table 6 Summary of calculation results for CHP system Ð option 2 Additional gas consumption by combined unit
Saving in electricity consumption by combined unit
Saving of Cost of Additional electricity gas consumed additional (kWh/annum) gas (£/annum) (kWh/annum) Day time 6,822,631
41,525
Night time
Maintenance Annual revenue Additional capital Payback cost cost saving cost of period (£/annum) (£/annum) combined unit (£) (years)
Cost saving Ð electricity (£/annum) Day time
Night time
1816607 550,795 90,830 14,871 16,098
48,079
202,510
4.21
Table 7 Summary of calculation results for CHP system Ð option 3 Additional gas consumption by combined unit
Saving in electricity consumption by combined unit
Additional Cost of Saving of gas consumed additional electricity (kWh/annum) gas (£/annum) (kWh/annum) Day time 4,992,109
29,441
Cost saving Ð electricity (£/annum)
Night Day time time
1,816,607 0
Maintenance Annual revenue Additional capital Payback cost cost saving cost of period (£/annum) (£/annum) combined unit (£) (years)
90,830
Night time 0
11,788
49,601
202,510
4.08
Prosser and Maidment [18] reported that this could be overcome if heat from the CHP unit is used to power an absorption chiller, which provides ÿ10 C glycol for cooling of the chilled-food cabinets. This novel con®guration provides a demand for the hot-water source, which allows the CHP unit to be fully utilised even in the summer months. An investigation to understand the feasibility and viability of this novel system was carried out and the results of this investigation are detailed in this section. 4.1. General description of the novel system A schematic showing the proposed system is detailed in Fig. 10. The CHP unit has been selected to satisfy the electricity demand of the Penge supermarket. This is used for lighting, equipment, the HVAC system and a vapour compression refrigeration system serving the frozen food cabinets only. The system is also con®gured to allow heat produced by the CHP unit to be used for space heating, to provide hot water and to drive an absorption chiller to cool glycol for circulation to chilled food cabinets. The glycol temperatures assumed to satisfy the chilled cabinets were ÿ10 C ¯ow and ÿ6 C return [19], and the performance data used for this system are shown in the Appendix.
Fig. 10. Schematic showing of electricity-based CHP and absorption HT refrigeration system.
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Fig. 11. Relationship between COP of the chiller and BIN temperatures.
The absorption chiller selected to achieve these glycol temperatures was a bespoke single-stage ammonia/water chiller. The performance of the absorption chiller was obtained using manufacturer's data [20] and is shown in Fig. 11. To produce the required glycol temperatures, it was necessary to power the absorption chiller using medium-temperature hot-water (MTHW) at 124 C, which required the use of a high-temperature CHP engine. 4.2. Investigation 4Ðinitial analysis of the combined CHP/absorption system The viability of the system described above was investigated with a CHP unit with the following capacities at the design conditions [14]
Electricity output: Heat recovered into primary water circuit: Heat recovered from cool exhaust gas: Engine's gas-consumption:
251 293 68 711
kW kW kW kW
The thermal and electricity output for this con®guration and the corresponding energy demand was simulated using the computer model and the results from this are shown in Fig. 12. From this, it can be seen that the electricity demand is always satis®ed by the CHP output. Also, it can be seen that the heat supplied by the CHP Table 8 Summary of calculation results for CHP system Ð option 4 Additional gas consumption by combined unit
Saving in electricity consumption by combined unit
Saving of Cost of Additional electricity gas consumed additional (kWh/annum) gas (£/annum) (kWh/annum) Day time 5,294,105
31,590
Night time
Maintenance Annual revenue Additional Payback cost cost saving capital cost of period (£/annum) (£/annum) combined unit (£) (years)
Cost saving Ð electricity (£/annum) Day time
Night time
1,816,607 550,795 90,830 14,871 12,130
61,982
373,994
6
Fig. 12. Output/demand pro®les for `electricity-based' CHP/absorption systemÐoption 4.
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unit is fully utilised, although supplementary heat provided by the gas ®red boiler is always required. The economics of this option was investigated using a revised BIN model and the comparative energy consumption and costs for this scheme compared to the traditional format are shown in Table 8. This shows a payback period of 6 years is produced for this schedule and the primary energy consumed is 20% less than the traditional system. 4.3. Investigation 5Ðapplication of absorption chillers for frozen cabinets The use of an absorption chiller for providing glycol to satisfy the frozen food cabinets has been considered and was found not to be practical. Whilst absorption chillers are available which can operate at temperatures down to ÿ60 C [20], these require unacceptably-high water-temperatures which are not available with current small-scale CHP systems. For this reason, this option has not been considered further. 4.4. Investigation 6Ðfrozen food cabinets with integral cascade condensers The feasibility of using chilled propylene glycol as a heat source for frozen-food cabinets employing integral refridgeration units with cascade condensers has been investigated and the results indicate that the advantage of this is small. Estimates indicate that the payback period would be similar or slightly higher than that calculated for the systems analysed in Section 4.2. Furthermore, the cascade system would only operate when there is a chilled glycol supply. During maintenance peri-
Fig. 13. Relation between cooling duty and relative price of absorption chillers.
Table 9 Summary of payback periods and primary energy consumption for all options Options
Traditional system
1
2
3
4
Payback period (years) Primary-energy consumption (kWh/annum)
± 7,892,821
8.98 7,275,492
4.21 7,951,445
4.08 7,674,992
6 6,422,900
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ods, the frozen-food system would not be able to operate without the chilled food system running. For this reason, this option is not recommended. 5. Conclusions The investigation has demonstrated that a conventional CHP system may be practically applied to provide heating and generate electricity in the supermarket. A number of options of conventional CHP have been investigated theoretically using a computer model of a small, Sainsbury's supermarket. This has shown that small primary energy savings are achievable and attractive payback periods are possible. Using an `electricity-based' con®guration, operating during trading hours only, saves approximately 3% of primary-energy compared to the conventional heat-andpower systems. For this system, a payback period of approximately 4.1 years has been calculated. Despite these advantages, a considerable quantity of heat is rejected to atmosphere with this system because this con®guration utilises the heat mostly for space heating, which is only required for part of the year. To increase the utilisation period, a novel CHP/absorption system has been investigated. This system uses a CHP unit to generate electricity for lighting, equipment and for a vapour-compression refrigeration system serving frozen food cabinets. As well as using the waste heat for space heating and to provide hot water, this system utilises the heat to drive an absorption chiller, which produces propylene glycol at ÿ10 C for cooling the chilled-food cabinets. This con®guration provides a continuous demand for the waste heat. This investigation has shown that this system is practical. However, in order to achieve the desired glycol temperatures, a CHP system which generates mediumtemperature hot-water is required. The investigation has shown this con®guration to be extremely ecient, as primary energy savings of 20% are achievable, as shown in Table 9. When taking into consideration the additional capital cost of this con®guration, a payback period of 6 years has been calculated. The main reason for this payback being slightly higher than that calculated for the conventional CHP system is the large additional capital cost of the absorption chiller. This is partly because the chiller selected is one of the smallest models in the manufacturer's range; this size of chiller was required because the typical store modelled was one of the smaller Sainsbury stores. For larger stores, better payback periods of between 4 and 5 years may be achieved as the unit cost of the absorption chiller reduces signi®cantly as the size increases as shown in Fig. 13. Also this equipment is currently designed and constructed on a bespoke basis, with increased demand and economies of scale further reductions in payback period would be realised. This investigation has subsequently been extended to consider the viability of incorporating an absorption/ secondary refrigerant system into the CHP scheme to provide the cooling for frozen-food cabinets. However, this con®guration has been shown to be impractical because absorption chillers capable of operating at frozen food temperatures require extremely high water-temperatures, which cannot be achieved with commercial CHP systems currently available.
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Acknowledgements The authors greatly appreciate the support of Sainsbury's Supermarkets Ltd, for this work and allowing the results to be published. The technical input of Dr Robert Tozer of Waterman Gore Ltd, Mr. J. Knowles of Air CHP Ltd, Peterborough and Mr. D.W. Maidment of ChemEng Ltd, Dartford is also valued. Appendix Table A1 Speci®cation for traditional vapour compression installation HT system
LT system
System description
Single-stage compressors with economisers, air-cooled condensers and direct expansion evaporators
Single-stage compressors with economisers, air-cooled condensers and direct expansion evaporators
Refrigerant Design condensing temperature Design evaporating temperature Design saturated suction temperature Design suction temperature Design COP Fan motor size condensers
R404a 45 C ÿ10 C ÿ12 C +10 C 3 6.2 kW
R404a 45 C ÿ33 C ÿ36 C ÿ10 C 1.85 1.6 kW
Table A2 Speci®cation for installation utilising CHP and absorption HT system System description
Absorption refrigerant Secondary re®rgerant Glycol-¯ow temperature Glycol-return temperature
Central heat driven absorption chiller connected to dry coolers and providing chilled glycol for circulation to the chillfood system Ammonia/water Propylene glycol ÿ10 C ÿ6 C (continued on next page)
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Table A2 (continued) Design dry-cooler ¯ow temperature Design dry-cooler return temperature Design COP Fan motor size of dry cooler Dry cooler water-pump motor size Glycol-pump motor size LT system System description
Refrigerant Design condensing-temperature Design evaporating-temperature Design saturated suction-temperature Design suction-temperature Design COP Fan motor-size condenser
30.5 C 33.5 C 0.51 6.6 kW 5.76 kW 4.5 kW Single-stage compressors with economisers, air-cooled condensers and direct expansion evaporators R404a 45 C ÿ33 C ÿ36 C ÿ10 C 1.85 1.6 kW
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