Biomass boilers

Biomass heating technologies range from log-burning on open fires, to wood log or pellet stoves and to more sophisticated automated biomass boilers. Pellet stoves are available at small scales, in the 5 -15 kW size range, whereas biomass boilers are available at larger sizes suitable for large homes. Both technologies can provide heat conversion efficiencies in excess of 90%. Typical 5 kW pellet stoves cost around £1 500 and larger biomass boilers cost £200 to £250/kW thermal output.

Wood pellet is the fuel of choice at the domestic scale as it is dry and has higher energy density than wood chip (which is not used at scales below 30 KW). Because of its finer size, wood pellet also flows better from hopper into the burner. However, wood pellet is expensive at £150 to £200 per tonne, and so domestic biomass heating struggles to be competitive with gas- or oil-fired heating.

Ground Source Heat Pumps

Ground source heat pumps collect low-grade heat from the ground and increase it in temperature via a heat pump to supply heating loads. The ground source heat pump system consists of ground loops, which are loops of pipe buried at shallow depth in ground adjacent to the building and a heat pump, which is installed in the property replacing the boiler.

The average ground temperature just below the surface in the UK is between 8°C to 13°C and remains constant throughout the year. Water circulated through the ground loops is at lower temperature than the surrounding ground so warms up slightly, picking up energy from the ground. In the heat pump, this low-grade heat is transferred to a refrigerant which is then compressed to increase the temperature, allowing a supply of heat to the building at approximately 50 - 55°C.

The temperature of heat supply from a heat pump is ideally suited for use with a wet underfloor heating system, but is lower than that traditionally provided by a boiler for a radiator based system. For this reason, it is important to plan for the installation of a ground-source system early in the design of the building's heating system, so that appropriate emitters can be installed for the temperature of supply.

Ground source systems are well-suited to new build energy efficient housing. They are less well-suited to retro-fit to existing dwellings as often the temperature of heat supply will be too low for the heat distribution system in the building. The heat pump itself is electrically powered so a reliable electrical supply to the building is required.

A typical domestic ground source system will have a heat output of around 5 kW and cost of approximately £1,250/kW.

Air Source Heat Pumps

Even old air is full of energy and air source heat pumps use the freely available heat in the ambient air to provide efficient heating at extremely low temperatures in excess of -10C. Air source heat pumps can be installed indoors or outside and provide considerable benefits to installation and they do not require any underground work.

A heat pump's refrigeration system consists of a compressor and two coils made of copper tubing (one indoors and one outside), which are surrounded by aluminum fins to aid heat transfer. In the heating mode, liquid refrigerant extracts heat from the outside coils and air, and moves it inside as it evaporates into a gas. The indoor coils transfer heat from the refrigerant as it condenses back into a liquid. A reversing valve, near the compressor, can change the direction of the refrigerant flow for cooling as well as for defrosting the outdoor coils in winter.

When properly installed, an air-source heat pump can deliver one-and-a-half to three times more heat energy to a home than the electrical energy it consumes. This is possible because a heat pump moves heat rather than converting it from a fuel, like in combustion heating systems. Coupled with a good domestic hot water tank the right air source heat pump will provide all space heating and domestic hot water for a new residential property all year round at temperatures greater than 0C and can easily be boosted for those days when termperatures drop below freezing. In all cases there should be no need for any additional boilers or heating systems.

A typical 6 - 8kW system costs £6,500 - £11,500 (not including the price of distribution system). This can vary with property and location. On average you can save £650 - £750 on your heating bills and 4.5 - 5.5 tonnes of C02 per year. Enviko sources all its heat pumps from reputable manufacturers including Thorén, Nibe and other top leading names.

Water Source Heat Pumps

Water source heat pumps work in a similar way to ground source systems, except that they use open loop collectors where underground water is circulated through pipe loops. The efficiency of such systems can exceed 500% due to the relatively high temperature of the underground water.

A water source heat pump can be used if you have a well, pond, stream or lake. In this case the water is drawn up directly to the pump's heat exchanger where its heat is extracted and the water is returned to the source.

Absorption Heat Pumps

Absorption heat pumps are essentially air-source heat pumps driven not by electricity, but by a heat source such as natural gas, propane, solar-heated water, or geothermal-heated water. Because natural gas is the most common heat source for absorption heat pumps, they are also referred to as gas-fired heat pumps. There are also absorption coolers available that work on the same principal, but are not reversible and cannot serve as a heat source. These are also called gas-fired coolers.

Residential absorption heat pumps use an ammonia-water absorption cycle to provide heating and cooling. As in a standard heat pump, the refrigerant (in this case, ammonia) is condensed in one coil to release its heat; its pressure is then reduced and the refrigerant is evaporated to absorb heat. If the system absorbs heat from the interior of your home, it provides cooling; if it releases heat to the interior of your home, it provides heating.

The difference in absorption heat pumps is that the evaporated ammonia is not pumped up in pressure in a compressor, but is instead absorbed into water. A relatively low-power pump can then pump the solution up to a higher pressure. The problem then is removing the ammonia from the water, and that's where the heat source comes in. The heat essentially boils the ammonia out of the water, starting the cycle again.

Rainwater Harvesting

Rainwater harvesting simply collects rain which falls onto a roof and stores it in a tank until required for future use. When the water is required, it is then pumped to the point of use, thus displacing what would otherwise be a demand for mains-water. A volume of water is kept out of the storm-water management system in the process, thereby helping to reduce flooding risks.

More recently, water usage has started to exceed supply, localised flooding has become an issue and water conservation is a requirement. Rainwater harvesting has an important role in alleviating these problems.

The systems work when rainwater is captured from the roof(s), and brought to a central point through the gutters, into a collection tank where it is filtered on entry. The size of the tank is dependant of the amount and purpose of the water but also of the annual rainfall and the size of the roof. A highly efficient and reliable submersible pump delivers the water to a service on demand. Where wished, or a special reason applies, delivery can be to a normal header tank instead.

The size of the storage tank is determined by considering the amount of water available for storage (a function of roof size and local average rainfall), and the amount of water likely to be used.