CO2 heat pump integrated thermal storage for domestic hot water in hotels

Abstract

Gas/diesel-fired water heaters are widely used in Indian hotels to deliver domestic hot water at 45–50 °C. These are substantial fossil energy consumers and pollute the environment. A CO2 (natural refrigerant) heat pump integrated thermal storage unit is an energy-efficient and eco-friendly solution for meeting heating demands. In tropical ambient conditions like in India, the energy efficiency of such heat pumps can further be improved when the heat source is the chilled water used for air conditioning purposes of the same hotel, thereby reducing the load on existing chillers. However, studies on the design of the proposed system and assessment of its potential for hotel application in tropical climates are still scarce. This information assists energy engineers and hotel management in evaluating new buildings or refurbishments. This paper demonstrates a design strategy to retrofit the conventional system with the heat pump integrated thermal storage unit and assess its energy, economic, and environmental benefits for a typical Indian hotel. Four different heat pump configurations, simple and ejector-assisted transcritical CO2 systems with and without internal heat exchangers, are analyzed and compared. The ejector-assisted CO2 heat pump without IHX having the minimum heating capacity (operates all time) integrated with a thermal storage tank of maximum capacity is observed to be an economical combination. In Chennai, where the average annual water temperature is 28 °C, the operating cost of the proposed system is only 7 % of that of a diesel-fired water heater. Also, CO2 emissions are reduced by 80 %. Thus, the proposed system is seen as an efficient and clean alternative solution for providing hot water and free AC in hotels.