As one solution for the climate crisis, carbon dioxide (CO2) is increasingly used as a refrigerant in heat pumps to provide extremely high-performance water heating, meaning a lot more hot water for a lot less environmental impact. Let’s unpack why this is important and how it works.
Why is hot water heating important?
A typical new home will use 46% of all energy consumed for domestic hot water (DHW). This percentage only increases as other parts of the home become more efficient, i.e. better insulation, efficient space heating, LED lights, Passive House building standards.
Therefore, an efficient DHW system is essential for affordable living costs and reducing carbon emissions of homes. A 75% reduction in energy use for hot water equates to more than a third off the total running cost of an average home.
For example, a $3,000 total power bill per year will include approximately $1,380 in DHW. From that $1,380, if you subtract 75% by using high-performance heat pump hot water, the DHW cost will be just $345 DHW. This means that the new annual power bill would be $1,965, with carbon reductions of around 360 kgCO2e/year.
Apartment buildings and other commercial buildings with showers or large kitchens will have a very large proportion of total energy devoted to water heating. Water heating efficiency in these buildings, therefore, becomes a significant issue for operating expenditure and carbon emissions.
How do refrigerants work?
All hot water heat pumps use a refrigerant that can change between liquid and gas states at different temperatures and pressures than water. This enables heat to be harvested from the ambient air at (almost) all times and transferred to water at temperatures that would not be possible if just using water. For example, CO2 refrigerant can absorb enough energy from air as cold as -25°C to heat DHW to at least +60°C.
Different refrigerants work better or worse at different air temperatures. CO2 is relatively unique as a refrigerant because the air temperatures at which it can operate range from -25°C to +45°C. There is almost no loss of performance between 1°C and 19°C ambient air temperature (COP of 4.3 to 4.7, or expressed differently 430% to 470% efficient). This is really important in the temperate NZ climate, covering the vast majority of operating conditions, especially if a daytime timer is used.
CO2 has an enormous capacity to absorb heat and it is exactly this property which is causing so many problems with the climate crisis. An apparently tiny change in CO2 concentration in the atmosphere — 280 parts per million pre-industrial to 410 parts per million today — is having a devastating effect on the climate. And it’s exactly this immense ability to absorb heat that makes CO2 such an incredibly effective refrigerant for heating water.
What else is important to consider with refrigerants?
Every refrigerant has two figures associated with them; the ozone depletion potential and the global warming potential. We only need to think about the banning of CFCs because of the hole in the ozone layer and the incredibly high instance of skin cancer in NZ to understand the real-world impact of these numbers. Thankfully, almost all refrigerants today have an ozone depletion potential of zero so the hole is slowly closing and we may need to slip, slop and slap less in future.
Unfortunately, the same is not (yet) true for refrigerants’ global warming potential (GWP). This is a measure of how many times more potent the refrigerant is than CO2 (base case) as a greenhouse gas when released to the atmosphere. CO2 (R744) of course has a GWP of 1. Other common heat pump refrigerants are R417a with GWP of 2,346 and R134a with GWP of 1,300.
Therefore, if the heat pump ever fails and leaks refrigerant or is not appropriately disposed of at the end of life — both of which are very likely in our current regulatory environment — the refrigerant leaks to the atmosphere. Completely invisible, colourless and odourless refrigerants but definitely potent if they have a high GWP.
The problem is that all the potential carbon savings from using a more efficient heat pump water heater are then negated by the tonnes of CO2 equivalent that enter the atmosphere as the high GWP refrigerant. With highly renewable electricity generation in NZ this can be the equivalent of more than six years of using a standard electric element in a cylinder.
Not all hot water heat pumps are created equal; some can offer high levels of savings and care for the planet. Reclaim is at the forefront of minimising the running costs and environmental impacts of DHW in homes and commercial buildings.