Optimization of combined heating and cooling in Supermarkets

Transcription

1 Optimization of combined heating and cooling in Supermarkets Funder-Kristensen T. 1 ; Fösel G. 2 and Bjerg P. 3 1 Ph.d. Head of Public & Industry Affairs, Danfoss, Nordborg, 6430, Denmark. 2 Dipl.-Ing. (FH) Innovation Expert Systems & Technology, Danfoss, Nordborg, 6430, Denmark. 3 M.Sc. Lead Application Specialist, Danfoss, Nordborg, 6430, Denmark. ABSTRACT This paper describes the opportunities for a combined heating and refrigeration cycle applied in food retail applications. An energy balance shows the heating capacity of the system related to specific store need. The efficiency gain from running the system with respect to both heating and cooling is described theoretically and different refrigerants are evaluated. To evaluate the technology a case study on a medium sized supermarket with CO2 as refrigerant is presented. Measurements of COP are described and evaluated with respect to influencing parameters. KEYWORDS Refrigeration, Heat recovery, CO 2 refrigerant, Supermarket INTRODUCTION The usage of high GWP HFC refrigerants is under increasing legislative pressure. Especially R404A has been identified as a substantial contributor to total HFC emissions due to its high GWP (3920) traditionally high charge sizes and leakage rates [1]. The alternative refrigerants solutions will depend on various factors as climate conditions, service skills, availability of low GWP refrigerants and of course cost, so there is no one refrigerant fits all solution. Heat recovery from commercial refrigeration systems has gained an increased interest during the last years. The principle of heat recovery is old but a systematic approach to utilize both the high and the low temperature sides of the gas compression system has rarely been seen until recently. Especially with the entrance of CO 2 as refrigerant new ways of improving efficiency and cost are becoming evident [2,3,4]. Looking at the development of CO 2 refrigeration systems for supermarkets one may state that the journey started with struggling with reaching HFC comparable efficiencies for cooling and after overcoming this challenge suddenly became an obvious candidate for free heat recovery due to its high heat capacity at high discharge temperatures. THE POTENTIAL OF HEAT RECOVERY The heating need in a store can be compared to the heat available from the refrigeration system using. Heat Recovery Fraction HRF = (1)

2 where is the Heat Recovery Fraction, & is the cooling power, 123 the Cooling COP, & '( is the heat needed from the refrigeration system to heat space, & -. is the heat needed to make hot water If HRF > 1 then there is sufficient available heating energy provided the actual needed heating temperatures can be achieved. If sufficient high temperatures can be achieved is a question of selecting a suitable refrigerant and running the system in an appropriate way, and of course at the same time obtain the best performance (COSP- see eq. 5) of the total system. An example of the Heat contributions of the HRF calculation is shown in figure 1 and related to the case study described later.. A HRF value of 2,6 is obtained which shows that the supermarket refrigeration system is very suited for heating for this particular store. In this case the heat loss to the ambient is assumed to be based on a heating loss factor of Figure 1. The energy balance 150 kwh/m 2 /year, the store size is 1000 m 2. The heat from electrical installations (light, rail heat, fan motors and other electrical devices as ovens etc.) is significantly high due to a relatively large bakery. The ventilation loss is assumed zero in this case where incoming cold humid air equals outgoing warmer but dry air. The amount of surplus heating energy depends on the climate conditions. This is mostly reflected in heat losses to the ambient (Q trans and Q vent ) and the amount of moisture which condensate in the cooling cases. Never the less, with a HFR value of 2.6 the loss of potential heating energy is significant, even in a cold climate in the winter. The excess amount of heat not used in the store can potentially be exported to other consumers nearby the store. COP DEFINITIONS AND MEASUREMENTS There are different Coefficient of Performance definitions in a CO 2 system with more evaporator temperatures. In the case of a CO 2 trans-critical booster system (two stage system with bypass) the following definitions are applicable & = 92; ; = & 8-93; ; >??@ = & 4-6& 8-94; where COP LT is the COP of the low temperature part of the system (freezing cabinets), COP MT is the COP of the refrigeration part of the system (refrigeration cabinets), Q LT is the Cooling power in the Low temperature cycle ; W LT is the compressor power in the low temp.cycle, f LT is the fraction W LT / (W LT + W MT ), Q MT is the Cooling power in the refrigeration cycle ; W MT is the compressor power in the refrigeration cycle, f MT is the fraction W MT / (W LT + W MT ). COP Cool is the Cooling COP for the total system. If the heating side is partly or entirely accounted as usable Energy the Coefficient of System Performance COSP is calculated as 12=3 = E F (5). E. F

3 CASE STUDY ON HEAT RECOVERY To investigate the consequences of installing a heat recovery system in a supermarket a study on a full equipped supermarket was done. The specific data for the installation can be seen in figure 2. Figure 2. Outline of a supermarket refrigeration system. Booster CO 2 (R744) with heat recovery The measurements of COP (see eq. 2 to 5) are based on the principle of utilizing existing standard equipment. These are of pressure and temperature transmitters in the suction and discharge lines combined with the in-duty compressor electricity consumption and compressor properties for swept volume. The selection of appropriate measured data in a field operating system has its own challenge. While most parameters can be controlled as long as the system is under laboratory conditions then the field operation conditions makes the selection of data difficult. Load and ambient conditions will change stochastically causing dynamic and thermodynamically unbalanced situations ions where a snap shot COP calculation can be very misleading. This means that trustworthy COP calculations have to be calculated based on averaged values over a certain time period. All COP values in this case study are calculated and evaluated if they are valid (each time compressors are shifted or pressure varies too fast, calculations can be in-valid). All values can be presented as 1 min, 1 hour or 24 hour running average as standard. The case study installation has now been running for more than a year and has been monitored with regards to COP as function of different parameters. To compare results with other installations relatively few studies on modern CO 2 systems are available but the results show good correspondence. [3]. In figure 3 (right) the heat used for hot tap water

4 (green), the heat used for space heating (black) and the energy lost to the ambient (blue) is shown together with the ambient temperature (red). The hot water usage remains stable during the week with a lower level during the weekend. Heat for space heating remains relatively stable during the week with 24 hours cycles. The heat rejected on the roof exceeds by far the combined usage of heat, so the conclusion is that there is plenty of heat available for supplying the store. This fits very well with overall energy calculations in the paragraph about HRF. The large fluctuations in the ambient heat rejection are Figure 3. Various COP s (left) and heating powers (right) during 1 week, Febr explained by compressor cut in and out. The store has a large bakery supplying to other stores and some days occupying large low temperature capacity for freezing bread. If ambient temperatures would drop lower i.e. -15 C the consumption of heat for space heating would raise around 30 % but still the majority of heat is used by the cooling and freezing cabinets. However, due to the properties of CO 2 a raise in the condensing temperature would be easily achievable and thereby a higher degree or up to 100% of the hot trans-critical gas heat could be utilized. In figure 3 (left) the COP of the low temperature (COP LT ) and the medium temperature cycles (COP MT ) are shown together with COSP. COP LT with an average value around 2,2 is close to constant during the week with a slight increase as outside temperature drops. The reason is believed to be the reduced humidity in the colder air which will also be reflected inside the store. COP MT with an average value around 3,7 is increasing more significantly as the ambient temperature drops. Again the

5 main reason is expected to be the decreased ambient humidity. COSP with an average value around 5,5 is increasing throughout the week mainly due to the increased COP MT as the heat recovery is stable. It is obvious that the theoretical limit ( COSP = 2*COP Cool + 1) is far from being achieved mainly due to unused heat still rejected to ambient. Figure 4. Running cost and CO 2 emission comparisons between R404A, CO 2 and CO 2 with heat recovery From the point of the store owner, the cost of running the store has been reduced significantly. The main reason is that the gas consumption has been 100% eliminated. Figure 4 (right) shows that thee cost has been reduced by more than 20%. In this case the economy calculations are based on electricity prices of 0,14 / kwh and gas prices of 1,40 / m 3 The environmental benefits are also high due to the eliminated gas consumption. Figure 4 (left) shows the emission reduction of the combined heating (including gas) and cooling processes before and after the heat recovery installations. A conventional R404A system is also put into this evaluation. The values for the R404A system are based on assumptions taken from. [3,4]. Adding heat recovery to a CO 2 booster system would decrease emission with around 7 % while comparing to a conventional R404A system without heat recovery the emission savings would be up to 34 % dependent on the amount of systems leakage (in this case leakage is assumed to be 10% per year at a charge of 200 kg, GWP of 3922, electricity counts for ½ kg CO 2 per kwh, Gas for 2 kg CO 2 per M3.) CONCLUSION Heat recovery within supermarkets has large unexploited potential which could be utilized for the benefit of retails owners and also external heat stakeholders. The heat recovery potential depends on the climate conditions where cold climates can benefit more than warmer climates. To evaluate the potential the heating factor is proposed. A case study of a supermarket has been presented. The case study shows that the heating needs of the supermarket in Denmark easily can be obtained and that excess heat is available if extra consumers where connected to the system. From process point of view the refrigeration cycle adapts to the heating needs. It means that within certain periods of the year the Cooling efficiency will be lower than compared to systems without Heat recovery. This penalty is however more than compensated cost wise and TEWI wise due to the avoidance of fossil fueled heating. During the cold periods the cooling capacity need of the refrigeration cases reduce significantly due to reduced ambient humidity content. This constitutes an even higher potential for heat production if the capacity could be used in a heat pump mode utilizing external heating sources

6 REFERENCES [1] Gluckmann et al ; SKM Enviros study : Phase Down of HFC Consumption in the EU Assessment of Implications for the RAC Sector; Ver. 11 Aug [2] Krieger T., Schryer R., Shuster M., Nugrohs S. Natural refrigerants for plug-in refrigerated Display Cabinets, Carrier Commercial Refrigeration, Lead Design Center, Proc. Of 23 rd IIR International Congress of Refrigeration, paper nr. 876, [3] Makhnatch, P. et al. Field Measurements, evaluation and comparison of supermarket refrigeration systems Final report Sveriges Energi- & Kylcentrum AB, IUC, Katrineholm, Sweden. [4] Mikhailov A., Matthiesen H.O., Suindykov S. Comparison of energy efficiency of systems with natural refrigerants, ASHRAE summer conference proceedings Ref number. D-ML-11-C015