White paper: Direct heat recovery from refrigeration and cooling systems vs geoscart ground coupling

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1 White paper: Direct heat recovery from refrigeration and cooling systems vs geoscart ground coupling Dr A. Leiper, N. Rivers and D. Zaynulin August 2014 Greeneld Energy Limited 1-2 St Andrews Hill London EC4V 5BY +44 (0)

2 Introduction Recycling waste heat is by no means a new concept. The world's rst commercial power plant, Thomas Edison's Pearl Street Station dating back to 1882, was a CHP plant that produced electricity and thermal energy simultaneously. Now, with a growing international emphasis on energy conservation, industry is turning to alternative sources of waste heat to satisfy loads such as space heating, domestic hot water (DHW) and process heating. One important source of waste heat is the cooling systems that chill or freeze food, provide comfort cooling or serve other cooling needs. By taking in heat at a low temperature and absorbing the energy used for the process, the waste heat is rejected at a higher temperature which can be reclaimed. As there is often a demand for heating in parallel with cooling, this solution is particularly attractive if the heat can be used immediately on site or nearby if buildings form part of a thermal energy network. A number of installations are already in operation that recover heat from retail refrigeration systems. However, recovering heat from refrigeration systems is very sensitive and poor design can impede the cycle eciency. This white paper aims to highlight the diculties and limitations associated with heat recovery from refrigeration. Thereafter, it presents the geoscart approach, whereby refrigeration systems are coupled to the ground via a water loop. This water loop exploits the ground as a large thermal energy storage bank and harnesses renewable heat from underground. Heat recovery applications in retail refrigeration Heat recovery systems employ a heat exchanger on the high pressure side of the refrigeration cycle in order to collect the heat rejected. In simple terms, the total heat rejected (THR, Q THR in kw) from a retail refrigeration cycle is the sum of the energy absorbed on the low temperature side (Q C, kw) and the work (W, kw) put in by the compressors: Q THR = W + Q C The refrigeration cycle can be represented on a P-h diagram as shown in Fig. 1. The values q and w are measured in kj/kg and denote the amount of heat and work per unit mass of refrigerant respectively, whereby Q = ṁq and W = ṁw and ṁ is the mass ow rate of refrigerant in kg/s. When heat recovery is employed, a proportion or all of the THR is recovered and any remainder is rejected to atmosphere as usual (Q R ). Typical heat recovery systems take heat from the discharge refrigerant vapour to provide DHW and then the heat of condensation for space heating. However, DHW and space heating supply temperatures are usually around 70 C and 45 C respecitvely, so in practice to meet these needs both the discharge and condensing temperatures must be increased. This comes at a cost to COP, as shown in Fig. 2. A combined COP of the whole system can be dened as: COP Com = Q H + Q C W 2

3 Pressure (bara) Head pressure Evaporating pressure 3 q C q THR w 1 - Compression 2 - Condensation 3 - Expansion 4 - Evaporation q THR - Total rejected heat w - Electrical work q C - Low temperature heat Enthalpy (J/kg) Figure 1: Simplied vapour compression refrigeration cycle P-h diagram High combined COPs can only be achieved when there is a favourable match between q C and q THR, and when q H is a high proportion of the q THR. If there is an increase in heat demand Q H the system can provide extra by raising head pressure, but before the COP and compressor isentropic eciency make the practice uneconomical. In heat recovery from CO 2 refrigeration systems, raising the head pressure will send the system transcritical due to the relatively low critical point of CO 2 (31.1 C). In certain cases this improves the system COP but still doesn't enable higher energy recovery. Alternatively the system can meet an increased heat demand Q H by increasing the mass ow rate of refrigerant. However, this will also increase the delivered cooling Q C which cannot typically be increased at will without a dummy evaporator. Such systems exist where the dummy evaporator cools air, turning the refrigeration system into an air sourced heat pump. However, these systems operate with very low evaporating temperatures, especially during the winter when ambient temperatures are low. This gives very low COPs, resulting in a large energy use overall since they will be used throughout the winter when heat demands are high. Pressure (bara) q R q THR q H Increased head pressure q C w Enthalpy (J/kg) Increased work Figure 2: Cycle P-h diagram when heat recovery is employed 3

4 The geoscart approach Geoscart is a geo-coupled heating and cooling solution. Low grade heat is recovered from the refrigeration cycle via a plate heat exchanger (PHE) in the proprietary Discharge Diversion Unit (DDU) and passed through the underground borehole heat exchanger network (Fig. 3). If the heating demand exceeds the amount of heat recovered, the decit is extracted from the rock's natural thermal energy resources. Alternatively, if there is a lower heating demand, the surplus heat is stored in the rock until required. The geoscart BHE network is made up of a number of proprietary coaxial BHEs that ensure highly ecient heat transfer with the ground. Heat pump Pump Plate heat exchanger DDU Air cooler Ground High pressure Low pressure VSD Compressor BHE network Figure 3: Simplied schematic of the geoscart DDU heat recovery system (patents pending) and ground loop The DDU allows for full control of the amount of heat that is recycled from the refrigeration cycle. If there is a need for heat, the ow rate of geoscart uid through the PHE is increased and the fan speed on the air cooled condenser/gas cooler is reduced. The DDU can be set to send the hot discharge refrigerant either to the air cooler or the plate heat exchanger rst, or to bypass one or the other of them altogether. The lower than ambient temperatures typical of the geoscart loop keep the refrigeration cycle head pressure low at all times, regardless of heat demand and season. A geoscart installation therefore improves the COP of the installed refrigeration system by up to 30%, provides a means of storing recovered energy in times of low heating demand (e.g. summer), and is capable of extracting geothermal energy to supply heat at excellent COPs (3-8 depending on conditions). Valve positions, uid ow rate and fan speed are controlled based on overall system performance, energy requirements, ambient conditions, geoscart loop temperatures and cooling cycle performance. 4

5 Summary Systems that recover heat directly from the high pressure side of refrigeration cycles are becoming more and more common, but most don't provide the energy savings they could and many are actually detrimental to overall cooling and heating COPs. Standard solutions not only lack capacity for storing rejected heat when there is no demand, but often operate at higher head pressures than equivalent systems without heat recovery, increasing energy use. When there is a high demand for heat the shortfall must either be supplied by conventional means (gas or electric, with high energy usage) or a dummy evaporator must be used to provide extra cooling load. Evaporating temperatures are often lower and condensing temperatures higher in such systems, giving poor eciencies and high energy consumption. A good match between the heating and cooling loads must be present to give acceptable eciencies. It is possible to overcome many of the drawbacks of typical heat recovery installations by coupling both the cooling and heating systems with a subsurface borehole heat exchanger network. The geothermal coupling addresses the dierences in heating and cooling loads by providing or storing surplus energy. Head pressure on the refrigeration system is kept low at all times and refrigeration COP is increased by up to 30% compared to a standard air-cooled system, thanks to the lower temperatures of the geoscart uid than those of the ambient, especially in the summer when cooling loads are higher. See the Geoscart eect on refrigeration white paper for more information. 5