Optimizing heat pump efficiency through dynamic temperature adjustments and variable electricity tariffs.

Last month, we explored the relationship between reducing the carbon content of electrical generation while simultaneously transferring heating demand onto the electrical grid as a two-pronged strategy for lowering our CO2 emissions. Heat pumps play a huge part in this strategy because of the multiplying effect they provide to the heat delivered, in comparison with direct electrical heating. However, they offer a further benefit in this complex scenario: the opportunity to use electricity when it is less in demand, by charging a thermal buffer, but to back off the load when demand for other purposes is high.

This requires a control system that can adjust the temperature targets in the heating system depending on the usage level of the electrical grid, but this is potentially easier than it might at first seem if the usage level translates into spot prices in a variable tariff arrangement. This, of course, is in reference to a large district heating system, which by definition, default, or design contains a very large volume of hot water. When the demand (and the price) for electricity is high, the flow temperature of the heating loop can be reduced. This causes the heat pump to reduce in capacity for a short time while the system rebalances itself, making it slightly more efficient, and, therefore, delivering more heat for a certain electrical consumption.

Conversely, when electricity is less of a constraint, the flow temperature can be raised, using the heat network as a giant thermal storage system. This reduces the efficiency of the heat pump, but in extreme cases where the proportion of intermittent renewable electricity in the mix is relatively high, the utility company may even pay you to take their kWh at certain times of day. Therefore, a lower efficiency is not such a big issue, provided this way of operating doesn’t cause additional wear and tear on the plant or other adverse effects such as poorer reliability.

If the electricity tariff is variable (linked to demand), then the temperature setpoint of the heat pump can relatively easily be adjusted automatically as the price of electricity varies. A district heating network could be adjusted by as much as 36°F (20 K) in this way if the network was properly designed.

Refrigeration systems also have a role to play in softening the effect of intermittency on the grid. Again, this can be linked to a variable tariff, encouraging heavier use of the refrigeration system when power is cheap and backing off if the price gets too high. In general, the swings in temperature that can be achieved in this way are much less than in the district heating system.

The major pitfall to be avoided is the effect of variation in cold storage temperature on the quality of the frozen product. Provided the product is below 0°F (–18°C), the span of the variation in temperature generally has a greater effect on product quality than the actual temperature because it determines the way ice crystals grow and how salts congregate within the product. Frozen bakery products and prepared foods, such as cooked ready meals, are most suited to this approach; meat is rather less tolerant of temperature variation; and ice cream requires tighter temperature control at a lower temperature to ensure the texture is not impaired.

Despite these constraints, the concept has been widely tried in Europe in recent years and has shown promising results when linking storage temperature and electricity tariff variations. The combination of increased load on the network and increased variability of the sources of electricity due to the effects of wind and solar power makes this a promising future technology.