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

Highlights

• Modifications to the standard CO₂ booster system are studied.

• State-of-the-art integrated CO₂ system for supermarkets is defined.

• The CO₂ system is compared to alternative cooling and heating systems.

• The state-of-the-art integrated CO₂ system is an efficient solution for cold and warm climates.

Abstract

This paper investigates the integrated and state-of-the art features of CO₂ trans-critical booster systems. The main objective is to identify the most promising solutions in terms of energy efficiency impacts.

First, the performance of modified features and integrated functions have been compared with the standard CO₂ system and alternative heating and air conditioning solutions. Subsequently, the performance of the defined state-of-the-art CO₂ system is compared to natural refrigerant-based cascade and HFC/HFO-based DX and indirect refrigeration solutions operating in cold and warm climates.

The results indicate that two-stage heat recovery, flooded evaporation, parallel compression and integration of air conditioning are the most promising features of the state-of-the-art integrated CO₂ system. This compact and environmentally friendly system is the most energy efficient solution in cold climates, and is also an efficient solution in warm climates, with comparable efficiency to cascade and HFC/HFO DX systems, but with no existing or potential limitations.

Introduction

The CO₂ trans-critical booster system has become the standard supermarket refrigeration solution for new installations in some European countries, mainly the Scandinavian countries. This technology acceptance originated from the fact that the system outperforms the conventional synthetic refrigerant-based system in cold climates Finckh et al., 2011, Sawalha et al., 2017, Karampour and Sawalha, 2017. 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. The installation and operating cost of the system has become comparable to conventional solutions (Zeiger et al., 2016).

There are two main innovation approaches pushing and spreading the applications of this system further south of Europe and beyond. The first is integration of all thermal functions in one compact unit. Air conditioning and heating integration provides a compact “plug & play” energy system, using electricity as the source energy and CO₂ as an environmentally friendly refrigerant. Compared to a complete stand-alone system, the integrated solution requires fewer components and less refrigerant. This contributes to a considerable space saving. Furthermore, in larger supermarkets the integrated functions are provided by a set of compressors. This reduces the risk of service interruption in case of one compressor failure (compared to stand-alone systems). Finally, the integration provides better control communications between the HVAC&R systems.

The second approach is to modify the “standard CO₂ system” to increase its energy efficiency, focusing on warm climate operation. Flooded evaporation by ejectors, parallel compression, mechanical sub-cooling and gas cooler evaporative cooling are some of the most recently applied modifications introduced into the supermarket refrigeration sector. Some modification options including mechanical sub-cooling and evaporative cooling improve the gas cooling process and provide higher energy efficiency in warm climates, while features such as flooded evaporation improves the system energy efficiency in any climate.

The energy efficiency impacts of these modified features and integrated functions are presented and discussed by various researchers. The following are samples of the researches which investigate the positive impacts of single modifications; single or multi-ejectors Hafner et al., 2014, Gullo et al., 2017, Schönenberger et al., 2014, parallel compression Javerschek et al., 2015, Javerschek et al., 2016, Karampour and Sawalha, 2015, flooded evaporation Minetto et al., 2014a, Tambovtsev and Quack, 2007, mechanical sub-cooling Llopis et al., 2016a, Bush et al., 2017, evaporative cooling Girotto and Minetto, 2008, Lozza et al., 2007, and LT de-superheater (Llopis et al., 2016b).

The research on systems integration is reviewed comprehensively by Karampour and Sawalha (2017). To mention some of these research works, heat recovery is studied by Sawalha (2013), Reinholdt and Madsen (2010), Polzot et al. (2017), Nöding et al. (2016) and the integration of air conditioning is discussed by Hafner et al. (2016), Karampour and Sawalha (2016a), Girotto (2016) and Gullo et al. (2017).

There is a risk that this variety of choices creates confusion. A shortcoming in the research and literature is to have a holistic approach examining all the alternative cooling and heating solutions to ease the decision- making process for supermarket stake holders on what to implement as the essential and most promising energy efficiency measures in the supermarket thermal energy systems.

What distinguishes this paper from previous research is that all the established energy efficiency features and concepts are studied in order to define the state-of-the-art integrated CO₂ system. Subsequently, this system performance is compared to modern alternative refrigeration, heating and air conditioning systems. The comparison assumptions are based on updated improvements in the components, control strategies and safety of all the systems.

The objective of this paper is to study and define the state-of-the-art integrated CO₂ trans-critical booster refrigeration system. To fulfil this objective, three approaches are adapted:

First, the energy efficiency impacts of modified features are studied. Second, the performance of functions integrated into the CO₂ system is compared to alternative stand-alone heating and air conditioning systems. The state-of-the-art integrated CO₂ system is defined based on the results of these two steps. Finally, energy performance and environmental aspects of the defined state-of-the-art CO₂ system are compared to alternative advanced and modern refrigeration system solutions including HFC/HFO DX and indirect systems, and cascade ammonia–CO₂ and propane–CO₂ systems.

The structure of the papers is as follows: standard and state-of-the-art CO₂ systems and their alternative refrigeration solutions are described in Section 2. The modelling details including assumptions and calculation methods are described in Section 3. The first part of the research results is presented in Section 4.1; it includes the evaluation of the standard CO₂ system modified features in order to define the “state-of-the-art” CO₂ system. The second part of the results, presented in Section 4.2, discusses the comparisons between the state-of-the-art CO₂ system and the alternative refrigeration solutions.