Moving away from CHP

The Energy Assessment Guidance issued last October by GLA clearly highlights that a new era for our industry is starting. So far, the use of CHP (Combined Heat and Power) has been an easy route to pass the ‘be clean’ criterion. However, as we utilise more renewables, the electrical grid is becoming greener and less reliant on fossil fuels. This has resulted in a drive towards ‘all-electric’ solutions, which is already affecting several developments for which the planning application has been submitted from January 2019 onwards. In today’s blog post we are going to look at the implications of this shift.

 

First, let’s look in a bit more detail at the reasoning behind switching to all-electric designs.

The electrical grid carbon intensity is currently rated at less than 0.3 kgCO2eq/kWh, compared to circa 0.75 kgCO2eq/kWh in 1970, proving a significant reduction of its carbon footprint. This has been possible thanks to the conversion of coal-fed power stations to gas, and to the increase of renewable power generation installations.

In practice, adopting an all-electric approach would still include a range of possible technologies, e.g. VRF systems, electric instantaneous hot water production, electric heating, and heat pumps of various types, either individual or communal.

Emissions

An important aspect to consider is that of emissions and air quality – an issue that is particularly crucial in London. CHPs allow for the decentralisation of the production of electricity, recovering energy (heat) that would have otherwise been dissipated in the environment and reducing the distribution losses associated with the electricity supply. The downside of CHP is the presence of exhaust gases that negatively affect the local air quality, which is already compromised in London.

In an all-electric scenario, the emissions are simply offset from the urban environment back to the power station; furthermore, additional energy must be produced to allow for the grid losses. The problem is therefore not solved, but it is just delocalised, unless the electricity is generated via renewables.

Space

It is also important to consider the spatial impact of CHP vs all-electric systems as this can have an impact on practicality and cost. The plant room for a CHP approach must be big enough to house thermal stores, boilers, expansion vessels and all associated ancillary equipment. In multi-unit residential developments, the plant room serves multiple dwellings and is usually located at ground floor or basement level.

An all-electric air source heat pump (ASHP) solution could require a larger plant area, both for the condensers (usually located on the roof) and also within each dwelling, i.e. in the resident’s MEP cupboard. Particular care must be also given to the maximum distances from the condensers to the internal units because this may limit options for locating the external plant; for example, plant could be required at more than one level.

Energy and Management

The two systems have different abilities to respond to the load fluctuations that are inevitably present in the energy demand. CHPs cannot easily modulate down, so they run almost at a constant speed, assisted by thermal stores to maximise their use and by boilers to cover the peak demands. On the other hand, Air Source Heat Pumps are not able to cover high peaks of demand, which means that the hot water production requires more time and a water storage.

Also, with ASHPs there are practical limits on the achievable temperatures (the maximum being circa 65˚C). A concern related to this is that the temperature could drop along the network due to heat losses, and therefore there are more chances to fall into the range of temperatures favourable to the growth of the Legionella bacteria.

Being both centralised options, the management aspects would offer the same beneficial/critical points. If an individual approach was used (e.g. one ASHP per apartment, located say on the balcony), this would be more preferable from the tenant/landlord’s point of view, as there is no communal pipework to be maintained and the system would be easier to be managed; however, in this case there would not be energy savings due to the diversity that can be applied among dwellings.

Future

Going forward, the critical points to consider are whether the grid is actually ready to sustain a significantly higher electrical demand. This in turn may have implications for the spread of energy storage on site, but a careful assessment must be carried out to understand the impact of the energy storage options – particularly their end of life dismantling.

The message for architects and developers for the coming years is that these different technologies may require some additional concessions compared to solutions adopted in the past, but the new options will improve the carbon footprint of the development, for everybody’s benefit.