Carbon dioxide refrigeration with heat recovery for supermarkets

Carbon dioxide refrigeration with heat recovery for supermarkets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . This paper describes the outcomes of a research project that investigates sustainable heating and cooling solutions for retail applications using a carbon dioxide (CO2) natural refrigerant (R744) for food refrigeration. The paper presents the findings from an applied research study on a booster CO2 (R744) system with high and medium temperature heat recovery. The paper includes a description of the conceptual design and a computer model along with its validation based on some experimental results. The energy consumption and carbon emission reduction are investigated using this novel system based on an existing supermarket as a case study. - I. Colombo*, G. G. Maidment, I. Chaer and J. M. Missenden Department of Engineering Systems, London South Bank University, 103 Borough Road, London SE1 0AA, UK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Abstract *Corresponding author. Received 15 November 2011; revised 8 March 2012; accepted 3 April 2012 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 INTRODUCTION Food is an essential of life. The food industry is a crucial sector for the balance of an economy, especially in our modern world. However, the production of food also impacts on our environment. Recently, Beddington [1] reported that a large proportion of carbon emissions is attributed to food. In the UK, a large proportion of this is due to the retail food sector. Refrigeration plays an important role in retail stores to maintain the food at the required temperature, but in doing so, it significantly contributes to greenhouse gas emissions both directly and indirectly. Greenhouse gas emissions can occur directly through the leakage of high global warming potential (GWP) hydrofluorocarbon (HFC) refrigerants used in refrigeration systems, which can be as much as 30% of the system charge per year [2]. Indirect emissions are also significant as these systems are large consumers of electricity and are reported to consume around 4 MtCO2e per annum, where CO2e is the carbon dioxide equivalent [3]. In addition to the costs associated with the leakage of refrigerants and energy, there are other reasons why reducing carbon emissions from the retail sector are important. In recent years, natural refrigerants have been proposed as an environmentally friendly solution for the refrigeration industry due to the unavoidable future phase-out of HFCs [4]. These refrigerants do not contribute to ozone depletion and have low GWPs. These refrigerants include ammonia, hydrocarbons and CO2. CO2 (R744) offers a long-term solution suitable for many applications in refrigeration and heating, from domestic applications using heat pumps to industrial and commercial applications. CO2 offers significant advantages as a refrigerant since it is non-toxic [5], nonflammable [6], environmentally benign (ozone depletion potential 0 and GWP 1) [7], has high refrigeration volumetric capacity [8] and has high heat transfer coefficients [9]. However, there are technical challenges to its application associated with its low triple critical points and high operating pressure [10]. The principle of a transcritical system is that sensible heat rejection occurs above the critical point at a constant pressure, resulting in gliding temperatures [10]; therefore, the refrigerant is not condensed by normal condensers or heat exchangers (HXs) but is cooled by gas coolers. Because of the low triple point, R744 refrigeration systems are suitable for freezing low temperatures (LTs) and chilling medium temperatures (MTs) at constant pressure. The combination of LT and MT in one system has been reported to be the most appropriate and efficient R744 application for supermarkets [11]. 1.1 Combined MT and LT booster transcritical system Most compressor manufacturers are now able to supply both MT and LT compressors with the same oil system, which makes booster systems possible [11]. The CO2 booster system includes the combination of MT and LT evaporators, receivers to separate the liquid and gas, gas bypass (BP) accompanied by expansion valves and a gas cooler. Madsen [12] describes the design, construction, installation and monitoring of a system similar to that installed in a small supermarket. The system is a booster system with 2108C MT, 308C LT, 35 bar in the receiver and 90 bar as discharge pressure, with 328C as the gas cooler exit temperature. The coefficient of performance (COP) values for the systems were not presented, but the electricity energy consumption was monitored and compared with that of stores using R404A [zeotropic blends [R-125/143a/134a (44/52/4)] systems; an R744/R410A [zeotropic blends [R-32/125 (50/50)] cascade system was also monitored. It was concluded that the transcritical system was more efficient than the R404A and R744/R410A systems by 4% and 2%, respectively. 1.2 Combined MT and LT enhanced booster transcritical system The principle of an enhanced booster transcritical system is explained by Javerschek [13] and its operation is same as that of a booster transcritical system. The difference compared to the booster system described above is that the gas BP instead of being throttled to the MT pressure, mixed and then compressed by a transcritical compressor, it is compressed directly to the high temperature (HT) pressure by another transcritical compressor in parallel compression. The Bitzer ECO compressor [13] can provide parallel compression as this compressor has two suction ports (a gas BP port and an evaporator port), leading to a common discharge (transcritical) chamber. Theoretically, Javerschek [13] type of systems has been shown to be more energy-efficient than the cascade and booster systems by 16% and 12%, respectively. The COP of the enhanced booster system can be improved by increasing the receiver pressure, which reduces the work of the parallel compressor. 1.3 Transcritical system with heat reclaim When operating transcritically, the discharge gas after compression can reach very HTs and this is a useful source of heat. This source of heat is valuable as (...truncated)