|As such are defined technologies developed to generate electrical and thermal energy out of Low & Zero Carbon sources located on or near a building site. Low & Zero Carbon technologies are described further in the European Directive 2009/28/EC and when related to buildings are assumed to supply them directly with energy.|
The most recognised Low & Zero Carbon options currently are:
In a bit further detail:
- CHP (Combined Heat and Power)
- CCHP (Cogeneration of Combined Cooling Heat and Power)
- Solar Thermal Water heating
- Wind turbines
- PV Panels
- Ground & Air Source Heat Pumps (GSHP & ASHPs)
Combined Heat and Power
Combined Heat and Power systems (CHP) generate electricity and recover a proportion of the waste heat for use in heating and/or domestic hot water. This results to overall efficiency being significantly greater than that of power stations linked to the National Grid. CHP plants typically use gas as the primary energy source and often incorporate a thermal store. Biomass fuelled CHP is possible however is typically sensible only for very large schemes due to operational difficulties with smaller capacity units.
CHP systems are typically considered only to be viable when running at least 5000 hours per annum and have an appropriate year round heating demand. In the UK such systems are usually sized to meet space heating and domestic hot water base loads i.e. 15% of the total load occasionally rising up to 45%.
Combined Cooling, Heat and Power
Combined Cooling, Heat and Power systems(CCHP) often referred to as trigeneration use the same principles as a CHP system. However the heat produced is also used to generate cooling via an absorption chiller. CCHP plants can make use of heat generated by the CHP plant in buildings with a cooling demand.
Similar to CHP plant CCHPs are typically considered only to be viable where running for at least 5,000 hours per annum and have an appropriate year round usage for the heat generated.
The absorption chiller incorporated within the CCHP often has a very low efficiency and the intention is for the CHP to be used to heat the majority of the domestic hot water base demand requirements.
Solar Water Heating
Solar thermal panels utilise the sun's energy to generate hot water for use within the building. Panels are commonly provided in either flat plate or evacuated tube arrangements. They are ideally located facing south at an approximate 30-60░ inclination angle (Europe) but may often be fitted vertically or horizontally if necessary.
Wind turbines generate electricity derived from local wind resources. Their performance depends greatly on the wind speed and turbulence that in turn is influenced by the terrain type and installation height.
In urban environments non-laminar wind flow is the norm due to turbulence created primarily by adjacent buildings with varying heights and geometry. There is growing evidence of urban wind turbines failing to perform in line with manufacturer's estimated outputs thus wind turbines are likely to produce only modest power outputs in urban landscapes and are usually considered non-viable when it comes to reducing a building's carbon footprint.
Photovoltaic (PV) panels utilise the sun's energy to generate electricity. The optimum orientation for PV panels is due South and approximately 30-35░ as an average inclination angle (Europe) provided areas considered are not subjected to shade.
Shading may become a detrimental factor in some cases in contrast to solar thermal technology due to the fact that PVs consist of cells connected in series and shading of a minor section may result to considerable performance drop of a whole panel and consequently drop the performance of a whole array.
PV panels may be integrated in all building structural elements even if orientation and inclination is not optimum. Return of investment is nowadays not the only driver as building environmental performance has become an emerging trend especially in the UK where the government has legislated heavily towards achieving relevant European environmental targets.
Biomass is considered to be a renewable fuel source as the CO2 absorbed during the growth period is assessed as being approximately equal to the CO2 emitted during combustion and hence deemed "carbon neutral". Biomass fuel is typically wood either in chip or pellet form.
Large fuel storage areas with associated transportation and delivery costs should be taken under consideration. It must be also recognised that such boilers also increase the NOx emissions when compared to gas fired boilers.
Air Source Heating and Cooling
Heat pumps comprise a relatively efficient way of using electricity to provide heating (cooling too). They use a refrigeration cycle to raise the temperature of the heating /cooling medium extracting /returning heat from to a certain source (e.g. the outside air).
The efficiency is higher if the temperature supplied by the heat pump is near that of the source. So heat pumps are often used with under-floor heating to provide a large area of heat emitter at a relatively low temperature (compared with radiators emitting heat from a boiler).
Ground Source Heating and Cooling
Ground source heating and cooling systems utilise the principle during heating mode of absorbing heat from the ground at a usable temperature and during cooling mode rejecting heat from the building into the ground. There are two primary methods which consist of utilising either open or closed loop systems.
Open loop systems extract water, typically from the chalk aquifer, and use it in either the heating or cooling process before rejecting this heat back to the aquifer through a separate borehole. Open loop systems in the UK require licence approval from the Environment Agency that typically has a 10 year duration.
Closed loop systems dissipate heat to or extract heat from the ground via pipework circuits that are typically inserted into vertical boreholes. These generally do not require Environment Agency licences as no extraction of water from the aquifer takes place.
Generally for both the open and closed loop options heat pumps are used in order to generate the heating and cooling water temperatures needed within a building.
Fuel cell technology essentially converts chemical energy into both electrical and thermal. Cells need to be continually supplied with hydrogen (derived from either a piped or storage source) and oxygen (derived from air). These are combined and the chemical reaction produces electrical energy, thermal energy and water vapour.
Fuel cells require a hydrogen fuel source that can either be from a piped source or from stored gas. However the more usual approach currently in the UK is to use natural gas in order to generate the hydrogen required to operate the fuel cell.
Fuel cells have various commercial and technical limitations. There is a high initial capital cost, only few established suppliers and very limited specialist design, installation and maintenance options. Certain fuel cell elements require regular replacement imposing a significant on-going cost implication. Fuel cells themselves are generally large, heavy and require fresh air ventilation.
It is important to consider when combining low & zero Carbon technologies that these do not cancel out each other's pros.