This paper describes the development of a CO2 air/water heat pump, designed to meet the tap hot water requirement of a residential building located in the northern part of Italy. The basic design consists of a single-stage piston compressor, a coaxial type gas cooler, an electronic expansion valve, a finned tube evaporator and a low pressure receiver. The heat pump is combined with a storage tank designed to maintain internal water stratification. A new control method for the upper cycle pressure was developed to maximise the COP of the heat pump, while the water mass flow is adjusted to maintain the set water temperature at the gas cooler exit. Before commissioning, the heat pump was factory tested to verify its energy performance and to validate the high pressure control logic.
CO2 single compression cycle is severely penalised when the hot sink temperature approaches the critical temperature; in this situation COP improvement can be obtained with a modified thermodynamic cycle. Parallel compression is a well known method to enhance efficiency and it was already considered as a possible solution for above 0°C supermarket refrigeration purposes. In this paper a theoretical analysis shows how it can actually be a realistic way for commercial refrigeration in hot climates. Technological aspects of multi-compressor systems with independent parallel compressors, as the volume ratio control between main compressors and parallel compressors are faced. The integration with water spray on gas cooler or evaporative cooling can further improve efficiency. Six-month data from a recent installation in Europe are presented, thus demonstrating that this solution is efficient, and it is also mature from a technological point of view so that it can be implemented when necessary.
The EU H2020 project MultiPACK has given the opportunity of installing three state-of-the art CO2 systems for supermarkets in South Europe, able to supply all the required thermal energy needs of the site, i.e. refrigeration, heating, cooling and hot water production. The MultiPACK units include parallel compression, ejectors for expansion work recovery and liquid recirculation and evaporator overfeeding, together with full monitoring of operation and performances. After more than one year of operations, field data of two supermarkets installed in Italy are presented and operating conditions and performance are illustrated; KPI indicators are evaluated to prepare for comparison with traditional solutions. A maximum monthly average COP of 4.2 was measured in central Italy in March 2020. The average specific energy consumption, due to Refrigeration, AC and heating, referred to the shopping area, was found to be 111 and 146 kWh m-2year-1 for the two sites.
This paper presents field results of a CO2 transcritical water chiller installed in a winery in North Italy. The unit is designed to cool down glycol, which keeps must temperature under control during fermentation; different water temperature level might be required according to specific step in the wine production process. Waste heat can also be recovered at high temperature, for cleaning purposes. A transcritical low pressure receiver design, with flooded, natural circulation evaporator is implemented. Data from the field are collected and analyzed to assess energy performance and useful effects under different boundary conditions and load requirements. Field data analysis, which are still ongoing, will allow for proper fine-tuning of the controller setup.
A recent development of CO2 booster commercial refrigeration systems, based on the adoption of ejectors for liquid pumping and suction vapour pre-compression in an economized cycle, has demonstrated to be a viable option to overcome the main issue associated with the use of CO2 in warm ambient conditions, thus promoting the diffusion of CO2 in Southern Europe. At least 20 systems using ejectors are in operation at the moment, with different layouts. Some of them incorporate both ejectors and auxiliary compressors. Data from the field are available to support expected performances; as a matter of fact, the adopted solution is gaining popularity. This paper summarises the experience acquired during the development of these innovations, also focusing on technological aspects, presents some results from the field and faces the issue of moving towards a simplified design. Simplification and standardisation of the high-efficiency CO2 solution are considered essential to successfully spread the proposal to countries where synthetic refrigerants are nowadays the first choice, also for economic reasons.
