What heat are they pumping? On heat-pumps and zero-carbon homes
You can’t skim through energy-related news without stumbling upon something about heat pumps. Here’s why that matters, in 9 points:
Space heating and cooling account for approximately 40% of household energy use. Traditional furnaces and gas heaters make Zero Carbon Homes an elusive dream. In California, for example, using gas to heat homes and buildings is a significant source of climate pollution, nearly matching the greenhouse gas emissions from all of the state’s power plants.
- Heat pumps are an energy-efficient way of electrifying heating and cooling tasks. Homeowners who opt for a heat pump can significantly reduce their carbon footprint and overall energy consumption, thus lowering their utility bills. Evaluations suggest that heat pumps can cut the energy needed for space and water heating by a factor of 3 or 4.
- Pairing a heat pump with a solar system, especially a battery-coupled system, can increase the self-consumption rate, drive down the effective levelized cost of energy (LCOE), and achieve energy-cost savings of up to 90%.
- How does it work? Heat pumps leverage phase transitions. An air-to-water heat pump pulls warmth from the surrounding air, compresses it to increase its temperature, and then pumps this heat through condenser coils that transfer heat to the water in a storage tank and throughout the system. See the figure and the resources below for a deeper look at the physics of this process.
Can heat pumps also heat in cold weather? Yes, even when the ambient air temperature is as low as minus 20 Celsius, there’s enough latent heat in the air to enable the cycle. Many households in Europe and the US (even in the coldest climates like Alaska, North Dakota, and the Northeast) use air-to-water heat pumps even during severe winters. Note, however, that more energy is required to operate in these conditions. Many heat pumps are “hybrids,” meaning they also have electric heating rods that are used in particularly cold temperatures. The operation range is not endless, though, and heat pumps can experience issues such as icing in cold temperatures, which can ultimately damage the system. Auto defrosting is available in some new designs. Recent research investigates newer mechanisms designed specifically for ‘super-cold’ regions.
- Can heat pumps cool the house? In short: yes. HVAC systems are rated by their Seasonal Energy Efficiency Ratio (SEER) and Heating Seasonal Performance Factor (HSPF). SEER measures the cooling output and HSPF measures the efficiency of a heat pump’s heating mode. Note, however, that in higher temperature environments, AC systems may still be a more efficient cooling method compared to air-to-water heat pumps.
- How much electricity is required? While most of the energy used by heat pumps is taken from the environment (i.e., heat is moved rather than generated), the devices do require some power to run the electric motor for the compressor, pumps, and fans. In good conditions, one kilowatt-hour of drive electricity can generate around six kilowatt-hours of heat. An 80-square meter insulated apartment needs less than 1,000 kilowatt hours of electricity per year. In older, less-insulated homes in Germany, air-source heat pumps generate about three kilowatt-hours of heat from one kilowatt-hour of electricity, according to research by the Fraunhofer Institute for Solar Energy Systems.
What is the best time to run a heat pump and what is its optimal running regime? In short: it depends. Since heat pump systems usually feature a hot water reservoir, it is usually efficient to run the thermodynamic cycle when cheap, clean energy is available, e.g., when excess-PV is generated. Then, the pre-heated tank may be used throughout the day and night. So the optimal operation regime depends on the availability of excess-PV, on the outside and inside temperatures, and on the dynamic electricity tariff. This is where an energy optimization framework comes in handy, by automating these timing decisions.
- Furthermore, the flexibility baked into heat pumps’ concept of operation means that the utility or third-party aggregators can throttle heat pumps up or down depending on the needs of the grid. This capability is also integrated into SolarEdge ONE, through the DERMs/VPP platform. For the homeowner, this means even deeper savings.
Missing an important point? Have an interesting angle? Please share in the comments.
Sources (images are all adopted):
· https://www.sierraclub.org/compass/2017/07/what-heck-are-heat-pumps
