An aerial view of Chennai, India
An aerial view of Chennai. Photo credit: Mohamed Ashish Gafoor for Shutterstock.

Study Finds That Replacing an Indian Hotel’s Diesel Water Heater With a CO2 Heat Pump Could Cut Operating Costs by 93%

Moving from a diesel water heater to a CO2 heat pump could also reduce emissions at the hotel, located in Chennai, by 80%.

A new study has found that a hotel in Chennai, India, could cut its annual operating costs and CO2 emissions by 93% and 80%, respectively, by replacing its diesel-fired water heaters with CO2 (R744) water-source heat pumps paired with thermal storage tanks (TST).

An unnamed hotel equipped with a diesel water heater provided the data for the study, with the authors noting that fossil fuel water heating equipment is the norm for hotels in India, with very few using heat pumps.

The study proposes a new approach: using water from the hotel’s air-conditioning system’s chiller as the heat source to enhance overall efficiency. The water from the chiller enters the evaporator at 12°C (53.6°F) and exits at 7°C (44.6°C), providing heat for the liquid refrigerant inside.

The study, “CO2 Heat Pump Integrated Thermal Storage for Domestic Hot Water in Hotels,” was written by a team of researchers, including M.P. Maiya, a Professor at the Indian Institute of Technology Madras, and Armin Hafner, a Professor at the Norwegian University of Science and Technology (NTNU), and published in the Journal of Building Engineering. The study is part of the INDEE+ project, an Indo-Norwegian initiative funded by the Norwegian Ministry of Foreign affairs to support India’s transition to natural refrigerants.

The researchers evaluated various configurations of CO2  heat pumps, including simple system and ejector-assisted setups with or without an internal heat exchanger (IHX). The study concluded that an ejector-assisted CO2  heat pump system without an IHX, combined with a large TST, offers the most economical solution for continuous operation in hotel applications.

Heat pump, TST sizing

In the Chennai hotel’s current setup, two diesel-fired water heaters produce primary hot water at 65–70°C (149–158°F). The primary hot water transfers heat to secondary water stored in two tanks, which is used in the 200 guest rooms and hotel kitchen. The daily domestic hot water consumption for the hotel averages around 40,000L (10,566gal).

“If the heat pump is designed based on peak heat demand, the heating capacity [would be] nearly 140kW (39.8TR) at a gas cooler water inlet temperature of 25°C (77°F), with the heat pump operating almost eight hours per day,” the study said. “During peak hours, the heat pump can serve the required demand, and surplus hot water can be stored during non-peak hours.”

The study found that with round-the-clock operation, the optimum heat pump size becomes 50kW (14.2TR). For this optimum capacity, a 9,000L (2,377gal) TST would be required to fulfill the hotel’s demand at a gas cooler water inlet temperature of 25°C (77°F) and outlet temperature of 50°C (122°F).

“A low-capacity heat pump demands a higher-capacity TST,” the study said. “However, considerable investment costs can be reduced with this integrated system.”

A rapid ROI

According to the study, a return on investment is possible in less than two years despite the higher upfront equipment costs due to India’s low demand for CO2  systems. The study noted that the relatively short ROI is due to a combination of the CO2  heat pump’s higher energy efficiency and because it makes the air-conditioning system more efficient by using its chilled water as a heat source.

While the main comparison was to diesel water heaters, the study also found that a CO2 heat pump paired with a chiller offered a lower operating cost than a liquefied petroleum gas heater, natural gas heater and hydrogen water heaters.

The OC of the CO2 heat pump and chiller combination was also compared to an R134a heat pump, and it outperformed it until the water inlet temperature was raised to 35°C (95°F). The study notes that the annual average water temperature in tropical Chennai is 28°C (82°F), while in northern cities such as Delhi it averages 20°C (68°F). 

Based on its findings, the study recommended that the CO2 heat pumps be installed in hotels in Chennai, Delhi and Goa. Goa was the site of a recent INDEE+ workshop on decarbonizing hotels using CO2 heat pumps and chillers.

“During peak hours, the heat pump can serve the required demand, and surplus hot water can be stored during non-peak hours.”

Hafner et. al

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