Hybrid refrigeration by CO2 vapour compression cycle and water-based adsorption chiller: An efficient combination of natural working fluidsFroid hybride par cycle à compression de vapeur au CO2 et refroidisseur à adsorption d’eau: une combinaison efficace de fluides actifs naturels

https://doi.org/10.1016/j.ijrefrig.2019.03.036Get rights and content

Highlights

  • Integrated hybrid system combining CO2 vapour compression cycle and adsorption chiller.

  • Efficient low temperature refrigeration using natural refrigerants only.

  • Dynamic optimisation of design and control to achieve maximum efficiency.

  • Energy savings up to 35% at high ambient temperatures and annual energy savings up to 22%.

Abstract

Sustainable refrigeration systems are of great importance to reduce greenhouse gas emissions. In particular, CO2 vapour compression cycles are very promising due to their environmentally friendly, natural refrigerant. However, a major challenge for implementing CO2 cycles is the low efficiency at high ambient temperatures resulting from high exergy losses in transcritical operation. To increase the efficiency, we present a hybrid system concept integrating an adsorption chiller into the CO2 cycle. The adsorption chiller employs the natural refrigerant water and is driven by waste heat from the CO2 cycle. The additional cooling generated by the adsorption chiller is integrated into the CO2 cycle to increase the efficiency of the overall hybrid system. Compared to a stand-alone CO2 cycle, we show by dynamic modelling and optimisation that the hybrid system leads to annual energy savings of 22% for a warm climate in Athens and of 16% for a moderate climate in Cologne. The results highlight the high potential of the hybrid system concept to efficiently provide refrigeration using environmentally friendly refrigerants.

Introduction

Refrigeration significantly contributes to global greenhouse gas (GHG) emissions, since more than 17% of world electricity consumption are used for refrigeration (International Institute of Refrigeration (IIR), 2016). While GHG emissions from electricity could be avoided by employing renewable energies, 14–35% of the total GHG emissions for refrigeration result from leakage of the refrigerants (Wu et al., 2013). Leakage has a high contribution to the GHG emissions due to the high global warming potential (GWP) of commonly used refrigerants (Beshr et al., 2016). To reduce the emissions from leakage, various natural low-GWP refrigerants are currently investigated (Ciconkov, 2018). CO2 is one of the most promising natural refrigerants because of various advantages: GWP of 1, favourable thermodynamic and transport properties, cheap and non-flammable, allowing low refrigeration temperatures (Bansal, 2012). However, a major drawback of CO2 as refrigerant is the low efficiency at high ambient temperatures due to transcritical operation. These low efficiencies result amongst others from high exergy losses of the non-isothermal heat release at high temperatures in the supercritical gas cooling step (Aprea et al., 2013, Aprea and Maiorino, 2009).

To increase the efficiency at high ambient temperatures, the transcritical operation can either be avoided (Sharma et al., 2014) or the exergy of the waste can be recovered (Sawalha, 2013). Transcritical operation can be avoided by cascade systems with a low and a high temperature cycle (Colorado and Velázquez, 2013). The low temperature cycle can be operated subcritically even at high ambient temperatures, since the high temperature cycle provides the heat sink for condensation. However, the high temperature cycle not only requires additional electrical energy but commonly employs refrigerants with high GWP (Sharma et al., 2014).

To reduce the consumption of electrical energy and to use environmentally friendly refrigerants only, thermally-driven sorption chillers have been proposed for the high temperature cycle (Fernández-Seara et al., 2006). Thermally-driven sorption chillers use heat to provide cooling and, thus, can substitute electrical energy by waste- or renewable heat (Deng et al., 2011). Compression-sorption cascade systems with CO2 in the low temperature cycle and sorption chiller in the high temperature cycle have been comprehensively investigated in the literature: There are theoretical studies proposing to drive the sorption chiller by waste heat, e.g., industrial waste heat (Chen et al., 2017, Colorado and Rivera, 2015, Jain et al., 2015). Other theoretical studies propose combined heat and power (CHP) systems to simultaneously provide mechanical power to the CO2 cycle and thermal power to the sorption chiller (Fernández-Seara et al., 2006, Garimella et al., 2011, Marimón et al., 2011). Experimental studies of compression-sorption cascade systems have explored solar and waste heat to drive the sorption chiller (Cyklis, 2014, Vasta et al., 2018). These experimental studies reached electrical energy savings up to 50% (Vasta et al., 2018). However, an external heat source has always been required to drive the sorption chiller.

Besides avoiding transcritical operation in the CO2 cycle, the waste heat can be recovered. The simplest way to recover the waste heat is direct heat supply, e.g., for heating of buildings (Nöding et al., 2016, Sarkar et al., 2008, Sawalha, 2013). However, finding a matching heat demand is challenging since the amount of waste heat is particularly high at high ambient temperatures. In contrast, the heating demand is generally low at high ambient temperatures. This mismatch between heat supply and demand motivates the use of a thermally-driven chiller, such as sorption chillers. A sorption chiller can recover the waste heat from the transcritical CO2 cycle to generate additional cooling. The additional cooling can be integrated in the CO2 cycle to increase the efficiency of the overall hybrid system (Graf et al., 2015).

Sorption chillers can be divided into absorption and adsorption chillers (Deng et al., 2011). Hybrid systems have been investigated with absorption chillers integrated into a CO2 cycle. Efficiency increases of 14–22% compared to a stand-alone CO2 cycle have been shown (Arora et al., 2011, Graf et al., 2015). However, absorption chillers also require electrical energy, which is often neglected when determining the efficiency. In some cases, the electrical energy demand of absorption chillers is only marginally lower than for compression chillers (Nienborg et al., 2017). Thus, the additional electrical energy demand can significantly reduce the efficiency increase by the hybrid systems.

Compared to absorption chillers, adsorption chillers offer some advantages: First, the electrical energy demand is lower for adsorption chillers (Helm, 2015). Second, adsorption chillers can employ low driving temperatures below 70°C (Choudhury et al., 2013), which enables high heat recovery rates from the CO2 cycle. Third, adsorption chillers commonly use environmentally friendly working pairs (Choudhury et al., 2013). Still, hybrid systems integrating an adsorption chiller into the CO2 cycle have only been investigated once in literature to the best of the authors’ knowledge. In a pilot project, Gerber (2011) has integrated a commercially available adsorption chiller into an existing CO2 cycle. This hybrid system achieved energy savings of 12% in the CO2 cycle at an ambient temperature of 25°C. However, the additional energy demand of the adsorption chiller was almost 8%, significantly reducing the net energy savings. Low net energy savings have been mainly attributed to a mismatch in size ratio between CO2 cycle and adsorption chiller and to suboptimal control parameters. Gerber (2011), therefore, conclude that system design and control is crucial and that there is optimisation potential in system design and control to achieve higher net energy savings.

In this study, we comprehensively assess the potential energy savings by a hybrid system integrating an adsorption chiller into a CO2 cycle. For this purpose, we present a concept for an integrated hybrid system. We employ dynamic modelling and optimisation to efficiently optimise control parameters, such as high pressure level and cycle time as well as design parameters, such as the size ratio between adsorption chiller and CO2 cycle. For the optimised hybrid system, we show the influences of ambient temperature and of electrical energy demand of adsorption chiller on the energy savings compared to a stand-alone CO2 cycle. Finally, we present annual energy savings for 2 case studies: the first case study represents a warm climate in Athens, where 22% of electrical energy can be saved. The second case represents a moderate climate in Cologne, where 16% of electrical energy can be saved.

The paper is organised as follows: In Section 2, we discuss hybrid system concepts and derive a concept for further investigations. In Section 3, we introduce the modelling and optimisation framework to systematically assess the hybrid system. In Section 4, we present the energy savings by the hybrid system and afterwards the conclusions in Section 5.

Section snippets

Hybrid system concept

In this section, we first discuss potential combinations of a CO2 vapour compression cycle and a sorption chiller. The discussed combinations generally apply for both ab- and adsorption chillers. For this reason, we refer to sorption chillers in general in the discussion in Section 2.1. Based on the discussion, we derive a hybrid system concept, which we present in Section 2.2. For the detailed presentation and the further investigations, we only consider adsorption chillers due to the

Modelling and optimisation framework

In this section, we present the modelling and optimisation approach to assess the hybrid system consisting of a CO2 vapour compression cycle and an adsorption chiller. All models are implemented in the object-oriented language Modelica (Modelica Association, 2017). First, we present the models of the CO2 cycle (Section 3.1) and of the adsorption chiller (Section 3.2). Second, we introduce the optimisation problem to identify the optimal control parameters and the optimal design (Section 3.3).

Results

In this section, we present the resulting energy savings by an optimised hybrid system compared to a stand-alone CO2 vapour compression cycle. In Section 4.1, we show the influence of the hybrid system design on the energy savings for different ambient temperatures.

In Section 4.2, we present the annual energy savings by the hybrid system for two case studies. The first case study represents a warm climate in Athens and the second case study represents a moderate climate in Cologne. Furthermore,

Conclusions

CO2 refrigeration systems become increasingly popular due to their environmentally friendly, natural refrigerant. However, low efficiencies at high ambient temperatures pose a challenge for the use of CO2 systems in warm climates. To overcome this challenge, we propose and model a hybrid system combining a CO2 cycle with an adsorption chiller.

In the hybrid system, the waste heat of the CO2 cycle is recovered to drive an adsorption chiller. The cooling power of the adsorption chiller is

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