Investigation of heat recovery in CO2 trans-critical solution for supermarket refrigeration

Abstract

PostprintThis is the accepted version of a paper published in International journal of refrigeration. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination. Tel: +46 (0)8 790 7889 Fax: +46 (0)8 204 161 E-mail: samer@energy.kth.se AbstractUsing computer simulation modeling this study investigates the performance of a CO2 transcritical system with heat recovery from the de-superheater. The influence of sub-cooling (or further cooling) in the condenser/gas cooler on system performance is investigated.Following the suggested control strategy in this study, the extra operating energy demand required to recover the needed heating energy from the analyzed CO2 system is smaller than what a typical heat pump would require for the same load. This is the case for almost all ambient temperatures over a full season. When taking the simultaneous heating and cooling loads into account, the CO2 trans-critical system has lower annual energy usage in an average size supermarket in Sweden when compared to a conventional R404A refrigeration system with separate heat pump for heating needs. CO2 trans-critical systems are efficient solutions for simultaneous cooling and heating needs in supermarkets in relatively cold climates. Keywords

Energy efficiency evaluation of integrated CO 2 trans-critical system in supermarkets: A field measurements and modelling analysis

“…This is why they have been popular in new installations in the relatively cold climate of northern Europe. The CO2 trans-critical systems offer another main advantage in cold climates, which is to recover all, or most, of the heat for space heating at relatively high efficiency (Reinholdt and Madsen, 2010) (Sawalha, 2013); therefore, it became a common practice in the past years to install CO 2 refrigeration systems with heat recovery.…”

“…The authors showed that by using gas cooler by-pass and optimal control strategy a significant saving in supplying the heating energy is achievable. Sawalha (2013) used computer simulation to investigate the performance of a CO 2 trans-critical system with heat recovery. A multi-step control strategy is recommended to maximize the efficiency of the CO 2 refrigeration system when recovering heat.…”

“…To cover this demand, some key parameters on the high pressure side are regulated by the heat recovery control system. The recommendations on how to control the high pressure side key parameters for efficient heat recovery have been discussed in detail in some other research works (Sawalha, 2013) and (Madsen and Bjerg, 2016). In brief, the recommendation consists of a stepwise control strategy; the first step is to regulate the gas cooler pressure P gc and keep the gas cooler exit temperature T gc,exit as low as possible by running the gas cooler at full capacity.…”

“…in equation (5)] is 85-105% at around mid-nights and 75-85% during opening hours of the supermarket. In some of the authors older studies, it has been shown that by using proper heat recovery control strategy, higher HRR (potentially up to 150%) can be expected from the CO 2 system with high heating COP (Sawalha, 2013) (Abdi et al, 2014). It is worth mentioning that there are always some heat losses to the ambient on the high pressure side of the system.…”

State-of-the-art integrated CO2 refrigeration system for supermarkets: A comparative analysis

“…This technology acceptance originated from the fact that the system outperforms the conventional synthetic refrigerant-based system in cold climates (Finckh et al, 2011) ) . The heat recovery from this system is an appealing choice to provide the heating demands of supermarket (Reinholdt and Madsen, 2010) (Sawalha, 2013). Furthermore, it avoids the ever-lasting concerns of following environmental regulations and restrictions.…”