Renewable Energy

The installation of a Renewable Energy system requires the skills of a Heating and Ventilation engineer. These are different to the skills of a Plumber and most problems stem from the incorrect installation. Unfortunately household-name boiler makers market their heat exchangers through the same outlets, so that plumbers see the basic technology at the builder’s merchants

With modern heat exchangers (a.k.a. heat pumps) energy can be extracted from a 5 0C difference to provide hot water at 55 0C. Obviously the daily atmospheric temperature varies so we need a supply of constant temperature to ensure that difference. That supply can come from the air or from the ground.

The kind of set-up is defined by the amount of heat to be exchanged:

• — Air Source 3-9 kwh
• — Ground Source (horizontal loops)9-22 kwh
• — Ground Source (vertical loop)22 + kwh

How much heat do we need? It is calculated based on the surface area of each area to be heated. It is the insulation factor of the building that strongly affects the heat requirement of a building:

• — Badly insulated100 w/m2
• — Average insulated (i.e. Building Regs) 50 w/m2
• — Well insulated 35 w/m2
• — Covered swimming pool 80 w/m2
• — Uncovered swimming pool150 w/m2

For example, a typical 3-bedroom house would have a floor area of 250 m2 and, with average insulation, would need 50 w/m2 x 250 m2 = 12.50 kwh, meaning fitting a couple of air source units or a horizontal loop system.

Air-source Heat Pumps are like air conditioners in reverse. They are relatively easy to install, needing only a south-facing location.

Horizontal loop systems are similar to the Slinky system, and need an open field. One acre can supply 16 kwh of heat and the pipes are buried at 1 m depth so that the field can be used normally. There have been incidents of the ground freezing when using the Slinky system due to excessive heat drain in areas where the pipes overlap or run close.

Heat can be exchanged from standing water (e.g. a lake or stream) but a minimum depth of 4 m of water is required.

Boreholes normally extend to around 100 m (depending on the drilling rig used) and will provide, for an 80 m active zone, about 10 kwh.

A heat exchanger for a borehole system cost, in 2009, around £17,000 and the insulated pipes to connect the system cost around £45 per metre.

Borehole systems are often installed because there is no field to lay out the horizontal loop. For a single house a borehole output is more than adequate. District Housing Systems are very functional.

The geological factors that affect the borehole systems are:

• — Rock strength;
• — Thermal conductivity and diffusivity;
• — Temperature and natural heat flow; and
• — Groundwater.

A geological investigation costs about the same as a 100 m borehole so the investigation becomes valuable when 10 or more boreholes are specified.