High energy efficiency for high-rise flats

SenerTec’s Gary Stoddart explains the role that combined heat and power (CHP) can play in increasingly popular new build communal heating schemes.

In communal heating schemes (sometimes referred to as community heating, district heating or heat networks), heat is supplied to individual properties from centralised plant, with the heat being delivered through a single pipework distribution arrangement. The concept is something that is being more widely seen, particularly in multi-occupancy high rise schemes such as apartment blocks.

Heat interface units (HIUs) – also known as heat boxes – will generally be used to provide the heating, or heating with domestic hot water, to individual properties. The householder will then control their supply with a room thermostat, a separate programmer or individual thermostat radiator valves. Importantly, HIUs also record the heat consumed for accurate energy billing; further guidance on metering can be obtained from The Heat Network (Metering and Billing) Regulations 2014.

In high rise developments centralised schemes can offer benefits over the more traditional solution of installing separate heating systems (which would typically comprise gas-fired combi boilers or a form of electric heating) in each individual property. For example, efficiencies can be increased and fuel consumption can be reduced. In fact, centralised plant systems are particularly well positioned to help developers meet their required targets in a cost-effective way. For more information, visit the BSRIA website (www.bsria.co.uk).

Using centralised plant also reduces the issues associated with supplying gas to multi-dwelling, high rise buildings. It decreases the risks surrounding gas distribution pipework and eliminates the need to fit numerous flue terminals, plume displacement kits and condensate drainage systems, hence reducing capital installation and whole life costs. Installing HIUs in each property rather than individual boilers further reduces installation costs and boiler size is also reduced, as individual boilers such as combis are generally oversized for the hot water capacity.

In the situation that the dwellings are to be rented rather than sold, it makes servicing and maintaining the heating equipment much easier, and removes the need for legally required landlord gas appliance checks to be carried out in each individual property (providing there are no other gas appliances in the dwellings).

In addition, with centralised plant, a number of different fuel sources can be used – and if low carbon technologies such as CHP are adopted, energy costs and carbon emissions can be reduced even further. In its 2013 document The future of heating: meeting the challenge, the Government suggested there is great potential to develop heat networks so that they can play a part in the move to low carbon heating.

CHP units generate electricity while also capturing usable heat produced during the process. On-site CHP is approximately 30 per cent more efficient than relying on traditional heating plant and electricity supplied solely from the grid, and can cost up to four times less, according to the Energy Saving Trust (www.energysavingtrust.org.uk).

When thinking about specifying CHP for a communal heating scheme one of the most important things to consider is that while there is no minimum operational period per annum that has to be met, savings will only be achieved if the CHP unit is running – so there must be a reliable, genuine need for the heat and electricity being produced (for new buildings, energy demand data can be obtained from design data or benchmark data from similar buildings).

Making the case for CHP should be based on a building’s individual energy needs, but the diverse thermal loads offered by multi-occupancy accommodation often present an attractive demand profile.

When SenerTec is assessing a site to see how feasible CHP is, the company firstly looks at the base load for heat and electricity. On a Dachs Mini-CHP unit the heat produced is between 12.5kW and 15.5kW (depending on the return temperature to the unit) and the electrical output is 5.5kW. If the base load is above these figures then the CHP will run continuously and the heat and electricity will be used in the building – which is the ideal situation.

The key to ensuring CHP delivers savings is to keep it as small as possible. Unfortunately in too many cases specifiers and consultants are being given poor advice, and the ‘10 per cent for luck’ rule is applied, meaning that many buildings have larger plant than required. If the heat demand is not present, oversized CHP will not run, and the anticipated electricity will not be generated.

A CHP unit supplements existing boilers and/or water heaters, so in a properly designed and commissioned system, the heating and hot water equipment within the building will draw on the CHP first, before demanding additional supply from the boilers. This ensures that the building gets maximum output from the CHP and the boilers only run for the shortest possible time – reducing gas consumption and carbon emissions as well as extending their operating life.

For a CHP installation to be successful, it’s fundamental for all parties to work together at the early stages of a project. It’s also essential for the CHP supplier to be involved throughout the design and installation process and beyond – if they can continually monitor the system after commissioning they can ensure the technology is being used correctly, continues to perform efficiently and delivers savings.

Consideration should also be given to long term maintenance. CHP engines are maintained on a running hours basis; for example the maintenance period for the Dachs Mini-CHP is 3,500 operating hours. If the CHP unit is monitored remotely through a modem it will inform the end user and the service team before a service is due.

In recent times there has been a trend towards centralised plant being used to deliver heating and hot water to individual properties in multi-dwelling developments such as apartments. This approach offers many benefits for developers and installation teams – including improved efficiency. Incorporating CHP into a centralised plant scheme means carbon emissions and energy costs can be further reduced.

Gary Stoddart is general manager at SenerTec.