Heat Pumps

A heat pump works like a refrigerator, using energy to move heat from a cold place to a warmer place. Heat sources include outside air, water or the ground. Ground source heat pumps extract heat from the ground for space and water heating. A closed-loop system collects heat by circulating a fluid in coils of pipes buried in the ground. An open loop system extracts groundwater and discharges it at a different temperature downstream. The ground has a near constant temperature making it more efficient and robust than a heat pump using the external air or water as a heat source. Can be used for cooling and storing heat from solar thermal systems.

Types

• Ground, water or air-source
• Various scales

Other components

• Ground source closed-loop systems require sufficient area for horizontal pipes laid beneath the ground or a number of boreholes (15 to 150m deep)
• Hot water cylinder (thermal store)

Typical saving

Efficiency varies. Typically, for every kWh of electricity put into it, a heat pump returns around 3 - 4kWh heat. Compared to electrical heating, it uses about a third of the electricity reducing CO₂ emissions to a third. CO₂ savings compared to gas-fired heating are smaller. Energy saving for cooling can be up to 90%.

Indicative cost

The ground works are the most significant element of cost for a ground source system, and vary significantly by site. Indicative cost of ground source heat pump system is in region of £1,000 per kW. Air source heat pumps are cheapest option, but less efficient. 

Main issues affecting feasibility

• Ground conditions: affect ease of construction and system performance. The presence of high groundwater levels is beneficial.
• Design of heating systems: Heat pumps are best suited to provide cooling or low temperature space heating, such as underfloor heating, as efficiency reduces as delivery temperature increases.
• Archaeological value or underground infrastructure: may prevent drilling.
• Long term performance: may reduce in efficiency if ground temperature reduces over time. Good design can avoid this.