Economic options for domestic hot water

Heat pump specialist Bob Long turns his attention to the economic potential of solar assisted heat pumps, also known as thermodynamic systems.

Due to slow growth within the heat pump market, it is not unusual for companies to extend their services into other renewable technologies. With electrical energy being the essential resource for operating a heat pump, solar PV is probably the most obvious choice of energy mix a company can offer.

Couple the production of electrical power through solar PV with an efficient battery system and inverter, and you may be able to make a significant impact on the carbon a household can actually save, or even make the first step towards off-grid living.

Staying on the subject of low carbon, it is worth looking at a range of heat pump products for the production of domestic hot water.

A number of companies have begun producing specialised heat pump equipment, targeting the economic production of hot water. The type of equipment I am referring to takes advantage of solar energy falling on a dark panel collector, from which the heat pump processes the thermal energy. These thermodynamic systems can effectively increase operating efficiency during days of sunshine, and reduce the overall cost of DHW production.

When the sun doesn’t shine, energy is still collected by the evaporator panel, although not at the same efficiency as provided on a warm sunny day.

MCS have recently provided criteria for this type of equipment to be eligible to qualify for the RHI, but the required seasonal performance factor of 2.5:1 is viewed by some in the industry as quite harsh, considering the parameters called for a water temperature of 55°C.

The seasonal performance factor for a standard heat pump to be eligible for the RHI payment is also 2.5:1, even though the COP for an air source heat pump is measured at +7°C air temperature, and +35°C output.

Many heat pumps that qualify for the RHI would not do so if they were solely dedicated to the production of DHW at continuously elevated temperatures, and indeed tested at the same parameters as a solar assisted DHW system.

The solar assisted DHW heat pump systems are however quite a good additional technology for the provision of DHW in summer periods when the main heating system isn’t required, and as such can produce attractive economics at the elevated water temperatures required for DHW.

Ideally, a mix of both technologies could provide the most efficient answer, although cost would obviously be a major consideration and actual water usage in the household must be considered in this calculation.

Many sales brochures indicate attractive payback times based upon unrealistic levels of water usage, when ideally these systems are most suited to larger families, with lots of clothes washing and showering taking place at regular intervals.

Solar assisted heat pump systems dedicated to the production of DHW generally have quite small power requirements and could be an option for off-grid users, where a PV array could probably provide enough energy to run the system on a daily basis.

The power requirement of the dedicated DHW system can be as low as 600 watts of electrical energy and a solar panel and storage battery bank would be capable of powering the system without an external power source.

This could be attractive to holiday homes, caravan parks and log cabin communities where the collaborative technologies of solar PV and a solar assisted heat pump provide a balanced and efficient solution to off-grid, or near off- grid living.