Domestic CHP (Micro-CHP)
(Including Fuel Cell Technologies)
Combined Heat and Power (CHP)
Introduction
With very few exceptions which are mainly related to renewable technologies, the majority of electrical power generated results in the production of heat. It is normal practice to make efforts to recover some of the heat to maintain generation at elevated efficiency levels, however, a disproportional amount of the energy involved in the generation process is simply treated as a waste product and dissipated into the environment, normally in the form of steam, resulting in an overall generation conversion efficiency range of 40%-50% for typical fossil fuel centralised generation plant in the UK.
In many countries the overall efficiencies of centralised electricity generation plant are improved through utilising what would otherwise be wasted heat to provide district heating schemes. Where heat from such plant can be fully utilised, combined efficiencies can be raised to around 70%. In order to achieve this, generation needs to be physically located close to areas of demand, resulting in plant often embedded into areas of dense population or industrial & commercial activity. This form of maximising efficiencies through not only utilising the generated electricity, but also the heat by-product is widely described as 'Cogeneration' (Cogen) or 'Combined Heat & Power' (CHP).
Micro-CHP
Recent developments in technology have enabled CHP to be employed in progressively smaller situations, from large industrial sites where the heat is utilised for both process and space-heating, through residential development sized systems where shared plant services various combinations of multi-occupancy buildings and individual residences, to a point where domestic-scale 'Micro-CHP' systems are appearing, providing individual properties with electricity, heating and storage of domestic hot water.
The 'Demand/Efficiency Quandary'
In order for any manufacturer, provider or scheme operator to make high efficiency claims for their offerings, the demand for heat must remain proportional to the demand for electricity, else heat will need to be wasted resulting in less efficient operation.
Reversing the position to where meeting heat demand results in CHP plant generating excess electricity to site/locality demand becomes less of an issue because excess power can be exported to feed the grid and used elsewhere, a solution not readily available with excess heat. This 'demand/efficiency quandary' becomes extremely interesting within the domestic CHP market when related to the various technologies which are beginning to appear.
Many of the currently available domestic Micro-CHP solutions offer little more than a heating source similar to a standard central heating boiler with a bolt-on generation unit which either utilises a proportion of the heat produced or energy extracted from exhaust-gas expansion to generate a relatively low proportion of electricity in comparison to heat, this ratio typically being around 6:1, therefore around 15% efficient generation of electricity.
A relative cogeneration ratio between heat & electricity generation of anywhere near 6:1 creates a seasonal demand imbalance issue. When generating electricity in the winter whilst heat demand is relatively high, the CHP solution overall efficiency could be reasonable, however, consideration needs to be given to relative inefficiency during the summer when there's little demand, other than DHW, for heat produced by the CHP system. If the solution installed is sized to meet average annual electrical demand, then the overall annual system efficiency can be ridiculously poor.
The 'demand/efficiency quandary' effectively explains why an effective CHP solution needs to maximise electrical output to meet demand whilst minimising heat provision to avoid waste when heating requirements are low, achieving this at high overall combined efficiencies too.
In short, in order to maintain operation at high efficiencies throughout the four seasons, a CHP system needs to provide an electricity generation to heat ratio which heavily favours the electricity generation.
