Heating and Hot Water in Typical Supermarket
It is predicted that the average global temperature could rise by between 1.5 and 4.5°K in the next 100 years. The principal cause of global warming is the emission of certain gases into the Earth’s atmosphere. These produce the ‘greenhouse effect’, 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 flooding 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. The basic argument in favour of CHP is that it is possible to produce heat and power more efficiently compared to the conventional methods of providing electricity, via the national supply grid, and heat, using a gas-fired 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 efficiency and high energy costs.
CHP schemes are an efficient 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 efficient 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.
The retail food industry is a large user of energy, consuming as much as 0.33% of UK’s total consumption. In the typical supermarket, electricity is consumed by lighting, equipment and the food refrigeration system. Energy is also consumed by gas-fired 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, its use in industrial applications in the UK is limited to large installations. However, in recent years, with the development of pre-packaged small-scale units with exergetic efficiencies often in excess of 50%, CHP has been successfully applied in new applications.
Previous work carried out by Maidment et al. 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 identified 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 defined. The method used to consider the viability of this system is then described and a number of configurations investigated.
To fully utilise the benefits 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 the HVAC system. Heat produced by the CHP system is used to power an absorption chiller to provide mono-propylene glycol at ?10°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.
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 floors. The ground floor includes the retail area, cold stores, food preparation/processing areas, dry stores and a restaurant. The first floor area includes offices, a staff restaurant and a kitchen. The store has been designed for approximately 700 people and has been assumed open from 7 am to 10 pm, 7 days a week.
The heating, ventilation, refrigeration, lighting and general electrical requirements of the typical supermarket are outlined below.
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-fired boiler provides hot water for the air-handling unit. Because the supermarket cabinets produce a large cooling effect, 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.
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. 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.
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. The condensers are fitted with head pressure control to ensure that the minimum condensing temperature is always maintained.
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