Hot water with a clean conscience

CO2 heat pump water heaters are helping Japan to reduce its CO2 footprint and its citizens to cut energy costs. And with clean energy high up on everyone’s agenda, the rest of the world looks set to take the plunge.

DATE 2023-11-28 AUTHOR Paul Redstone

It’s no secret that the Japanese love a nice hot bath. So much so, in fact, that water heating accounts for close to 30 percent of Japan’s total domestic energy consumption and has a significant impact on the country’s ability to meet its obligations under the Kyoto Protocol. In response, Japan has come up with the CO2 heat pump water heater – a solution that can significantly reduce a household’s energy consumption and CO2 output.

The solution comprises a heat pump and a hot-water storage unit. What makes it special is the use of CO2 as a refrigerant. Known as a natural coolant, CO2 has a lower GWP (global warming potential) factor than traditional CFC refrigerants. It’s also an excellent way to recycle a common waste product.

The Japanese government is a strong believer in CO2 heat pumps and sees them as an important contribution to fulfilling its Kyoto Protocol obligations, under which the country must achieve a 6 percent reduction in CO2 emissions, compared with 1990/91 levels, over the next five years. The government is providing grants to help householders purchase CO2 heat pump water heaters, with the goal of installing 5.2 million units by the start of the 2010 fiscal year. With this programme implemented, Japan will have achieved more than 10 percent of its total goal for CO2 emissions reduction by 2018.

But interest is quickly spreading beyond Japan, and more than 600,000 units have already been sold in Europe. There is enormous global market potential as demands for efficient energy increase and CO2 emissions requirements become ever more stringent.

Water can be heated to 90 degrees Celsius, with an energy saving of around 65 percent compared with conventional electric water heaters. It’s also around 80 percent cheaper to heat water this way than to use Japan’s town gas system. What’s more, by not burning fossil fuels to heat water, CO2 emissions from water heating can be halved.

A CO2 heat pump water heater comprises a heat pump and a hot-water storage unit. The components are connected in series, and the CO2 gas refrigerant circulates around the system. The system derives two units of energy from ambient air temperature for every unit of electrical power used, but more than three units of hot water energy are produced as a result. Energy for the refrigerant is collected from the external air via a heat exchanger. A centrifugal fan is generally used to provide air flow. The CO2 is heated to around 100 degrees Celsius under pressure of 10 MPa using a gas compressor, at which point it becomes a supercritical fluid. 

Energy from the heated refrigerant is then transferred into water via a heat exchanger, resulting in hot water. Water temperatures around 5 degrees Celsius and up are suitable at this stage. Ejector or expansion valves are used to reduce pressure on the refrigerant, allowing it to cool and revert to CO2 gas.

Some 25 companies currently manufacture CO2 heat pump water heaters in Japan, where the installed base had reached 2 million at the end of October 2009. Many are now planning to launch their units in Europe and the rest of the world.

 A wider market

Alfa Laval’s key product for the CO2 heat pump water heater market is the APX10 brazed plate heat exchanger. It’s ideal for this price- and space-conscious market, where it can replace larger, more complicated and more expensive heat exchangers. But Peter Nobel, general manager of the OEM Segment & Business Centre BHE, says the market for CO2 heat pump water heaters is just one slice of a larger pie.

“It’s a very big market in Japan, and potentially in Europe and the US, where more energy-efficient new house constructions offer potential for CO2 heat pumps,” he says. “But there are wider applications for CO2-related products. We are developing heat exchangers for CO2 applications that can handle larger capacities, and there is interest from the market. Applications include refrigerated transport and refrigeration systems for supermarkets.”

Development of these applications, Nobel says, is likely to be fuelled by political pressure to reduce the environmental impact.

Nobel says Alfa Laval has a great deal to contribute in this area.

“We have in-depth knowledge of heat transfer and provide extensive service and support around the application,” he says. “We also have a lot of experience in design work and prototyping.”