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CO2 refrigeration system with geothermal integration

EKA has evaluated the performance of CO2 refrigeration systems when combined with a geothermal storage function. EKA published in collaboration with KYS – the Swedish Refrigeration Industry Cooperation Foundation – a report which is based on three master thesis projects performed in the years between 2017 and 2019 in cooperation with KTH.

In 2017 a 1st phase analysis was conducted, involving 10 CO2 refrigeration plants with geothermal functions, installed in supermarkets. This preliminary analysis showed a systematic lack of measurement points in the installations, which is necessary for an energy efficiency assessment. For this reason, an instrumentation guideline was created and presented. Later on, in the years between 2018 and 2019, the project gained further access to several newly built installations in supermarkets and ice rinks. Among these, one supermarket was selected for an in-depth analysis. Additionally, two case studies were conducted to evaluate the economic benefit of the geothermal function in ice rinks utilizing CO2 refrigeration technology.

Operational strategies for heat recovery have also been deeply investigated. The best theoretical operational strategy was compared to the ones used in real conditions and the differences in the annual energy usage were assessed through modelling. The results show that an alternative to the best theoretical operational strategy does exist: heat can be extracted from the ground while low-temperature heat is rejected by the gas cooler. Such an alternative strategy has important technical advantages with a negligible increment of the electrical energy usage.

An important difference between the two applications studied, supermarkets and ice rinks, is the temperature level at which heat is utilized. In many supermarkets, the demand for tap water is marginal, while in ice rinks high-temperature (70°C) heat is an important part of the demand profile. For these reasons, optimal control of the heat discharge while respecting the required forward temperature and high temperature demand will also vary with type of application. A more complex heat recovery control algorithm for ice rinks was developed, tested and compared with the control currently utilized in one of the case ice rinks. The results show that the algorithm would lead to a 12% reduction in annual energy usage for the best scenario.

The benefits of geothermal integration were evaluated by applying the BIN hours method and utilizing typical Swedish prices. For a typical ice rink in Sweden the results suggest that if a CO2 refrigeration system is upgraded with a geothermal function, it can save between 1.7 and 6.8% of the annual energy consumption. In the best case, this study suggests that the geothermal function would pay back the additional investment cost in 16.4 years.

In the case of supermarkets, it was calculated that subcooling through a geothermal storage can save between 5 and 10% of the total electrical energy consumption, in a Stockholm climate. This represents almost the total amount of savings, since in winter the geothermal heat pump function is used only for peaks in heating demand. However, relying only on savings from subcooling was found not to be enough to economically justify a geothermal storage in a supermarket. For this reason, several scenarios were investigated to produce parametric curves and to perform a sensitivity analysis aimed at identifying the cases where this system solution is economically justified. These were identified to be supermarkets with a Heat Recovery Ratio (HRR) higher than the average, e.g. supermarkets supplying heat to the neighbouring buildings (assuming Stockholm climate and systems with an average HRR of at least 70% over the winter period).

Hours of operation under certain HRR range. Example from results for high activity ice rink 12-month season scenario.

Design and evaluation of these kind of systems is a complex task and involves in depth understanding of load profiles, refrigeration systems as well as the geothermal storage dynamic behaviour. EKA has in this research together with KTH gained a very specific knowledge which allows this techno-economical analysis to be done. Further considerations to be taken into account when designing refrigeration systems integrated with geothermal storages can be found in the full report.

Rapport CO2 & Geolager KYS 28SEP20 ENG c
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