By Jeremy Cook
Resistive electrified heating—where a current passes through an electrically resistive medium, shedding energy as heat—is the simplest manner of electrical heating. In fact, this heat shedding is inherent in electrical processes and is typically an inefficiency to avoid and/or manage with devices like heat sinks and cooling fans.
In a resistive heating role, this inefficiency (heat) is the product, making it theoretically 100% efficient on an energy output/input basis.
Unfortunately, 100% efficiency isn’t good enough to make electric home and building heating practical versus the combustion of fossil fuels. There is, however, another solution: the electric heat pump. Instead of converting electrical energy directly into thermal energy, heat pumps absorb and transport heat from one area (i.e., outside) and pump it indoors, multiplying the heat output versus the electrical energy input. Heat pump efficiency and cost savings versus resistive electrified heat can be quite attractive.
Heat pump performance and operation
Heat pump performance is measured by the system’s coefficient of performance (COP), expressed by the equation: work input x COP = heat output.
While a resistive heating coil’s COP is 1, heat pumps can achieve a COP of 2 or 3 (or even higher, depending on the design and thermal conditions). Divide the required heat output by the COP to give the required energy input—normally a fraction of the equivalent resistive heating input.
Heat pump energy transportation is accomplished via a thermal cycle, using a low boiling point fluid (a refrigerant) in a heat pump circuit. This is the same thermal cycle whereby a refrigerator pumps heat out of a confined space to keep food cold, but in reverse.
Simplifying the process for an indoor heating device:
1. A low boiling point thermal fluid (i.e., refrigerant) leaves the compressor as a high-pressure, high-temperature superheated vapor.
2. Fluid enters the indoor heat exchanger. Energy is lost from the fluid and added to the indoor air as heat (with forced air flow via a fan), causing the liquid to condense into a high-pressure, slightly cooler liquid.
3. The expansion valve transforms the fluid into a low-temperature, low-pressure liquid/vapor mixture.
4. Fluid enters the outdoor heat exchanger. Energy is added to the liquid from the outside (with forced air flow via a fan), causing it to evaporate. The fluid leaves the heat exchanger as a low-pressure, low-temperature, slightly superheated vapor.
5. Fluid enters the compressor, transforming into a high-temperature, high-pressure, superheated vapor, starting the cycle again.
The key concept in this cycle is that energy is lost to the environment at step 2, heating the indoor space, while energy is absorbed from the outside at step 4 and transported indoors. The same concept is performed in reverse to cool indoor spaces in the form of an air conditioner.
Refrigerants usually have boiling points well below 0º (F or C). R410A, for example, has a boiling point of -48.5ºC (-55.3ºF). The key to this cycle is that a large amount of energy input is required to boil a fluid, and a large amount of energy is released upon condensation.
Modern heat pump advances
The general concept of a heat pump has been understood for roughly two centuries and has been used in more temperate climates for home and building heating for many decades. Until recently, however, heat pumps have been impractical for use in colder climates like the northern US and Canada. Recent advances are changing this restriction.
High-performance refrigerants with very low boiling points (e.g., R410A’s -55.3ºC boiling point) allow for low-temperature operation, and improved heat exchanger designs facilitate gathering more heat from the outdoors.
Variable-speed compressors that use advanced motor drivers allow heating equipment to ramp up and down as needed, as opposed to being fully on or off.
Improved IGBT gate drivers and even silicon carbide-based devices, along with current sensing products that enable closed-loop control and advanced predictive heat pump maintenance, will help to advance this technology into the future. Other components like advanced magnetics and inductors, and even connectors, can help heat exchangers run at top efficiency with robust long-term reliability.
Future heat pump possibilities
This article focuses on heat pumps in the context of heating indoor spaces with outdoor air, but heat pump water heaters can use the same technology. These concepts can also be implemented using a buried heat exchanger in a ground source heat pump setup, taking advantage of the earth’s near-uniform temperatures to create a more readily available energy source than colder outdoor air provides. Heat pumps could also be employed for industrial processes, replacing fossil fuel or resistive options.
Given their advantages and the current push towards reducing fossil fuels, expect to see the expanded use of heat exchangers for heating soon. With better electrical and mechanical design improvements, we can also expect greater efficiencies out of these units, making them an even more attractive option in future years.
See related product
See related product
See related product
