District heating

Mike Hefford, Remeha CHP general manager

CHP in heat networks

Smaller-scale communal heat networks have a pivotal part to play in achieving UK carbon targets. Mike Hefford, Remeha CHP’s general manager looks at the benefits of applying CHP at the heart of the heat network and the role of suppliers in helping achieve optimum system performance

occupancy buildings. By capturing energy that is often wasted in power generation and supplying it to homes and buildings, heat networks achieve the twin goals of reducing carbon emissions from heating and lowering costs for consumers. The government outlined the vital role of heat networks in the long-term decarbonisation of heat in its Clean Growth Strategy. More recently, it has allocated £320m of funding out to 2021 to support public and private sector projects in England and Wales.

A Communal heat networks

So how do we define a heat network? There are two main types: district heating and smaller-scale communal heat networks. In both, heat is distributed from a central energy source in the form of steam, hot water or chilled liquids to supply heating, cooling or hot water.

While district heating networks may vary in size

and can cover an entire city, communal heat networks supply heat and hot water to multiple occupants from a central energy centre. Given that 11,500 of the 17,000 heat networks in 2017 were communal heat networks, these smaller heating schemes constitute an important part of the drive to reduce emissions cost-effectively.

CHP at the core

Communal energy centres typically comprise a number of components. While different heat sources can be used, one low carbon technology that can offer real, tangible benefits in a heat network is Combined Heat and Power (CHP). Remeha’s low NOx condensing CHP technology, for example, enables primary energy resources to be used up to 40% more efficiently than conventional methods of generating electricity and heat.

On-site generation benefits

How? Traditional power stations can lose up to two thirds of the fuel consumed as heat. Compare this

16 February 2019

dopting heat networks is widely recognised as one of the most cost-effective means of using energy more efficiently in multi-

all projects, good design is central to their success. So what considerations should be made when applying CHP in communal heat networks? The first step is to undertake a feasibility survey as buildings with high, constant demands for heat and electricity will reap maximum benefits from CHP. The second is to ensure accurate sizing to maximise the efficiency of the system. The thermal capacity should be matched to the thermal demand base load of the system to enable the customer to claim Enhanced Capital Allowances (ECA) on the equipment – note that this scheme is due to close in April 2020. CIBSE AM12 suggests that CHP is sized at 10-15% of the peak demand for both heating and hot water applications. We recommend matching the thermal and electrical base loads so that the building can use all the heat and electricity generated. Ensuring that the CHP can run constantly will maximise the efficiency of the system. For hot water only applications, a cold water pre-

The greater the difference between gas and electricity costs, the greater the savings when applying CHP in a heat network

with CHP which generates electricity on-site at lower gas prices to supplement or replace the electrical grid supply. It then recovers and re-uses the otherwise ‘waste’ heat to provide useful space heating or hot water in buildings, reducing carbon emissions by up to 60% compared with conventional methods.

With gas prices currently at around a quarter of

the cost of electricity, and electricity prices predicted to rise faster than gas, there are clear economic as well as environmental benefits from displacing carbon-intensive grid electricity with CHP electricity. At the same time, the local generation process

ensures a reliable, secure energy supply. This is particularly relevant as the electricity grid adapts to the changing energy mix and higher peak loads imposed by electric vehicles.

Optimising system efficiency

Communal heat networks support the drive to reduce emissions through improved energy efficiency, while reducing energy costs. But as with

heat system can be employed. However, a demand profile and buffering are key elements to ensure respectable financial payback. While CHP can be oversized to prioritise electricity

generation, this means that the heat will need to be ‘rejected’, reducing the efficiency of the system.

Supplier support

Experienced suppliers will be able to advise on electrical design considerations. This should include the size of the cables (to reduce the maximum volt drop), controls (to stop units from exporting to the grid), and the interfacing of the BMS. If sized correctly, there will be little need to export to the grid. However for smaller CHP units, it’s advisable to consult with the Distribution Network Operator (DNO) to see if the CHP can be connected to the grid and if they will accept export. A G99 (formally G59) application process will also need to be made – again, ask your supplier for assistance.

Optimising performance

Given that heat networks combine a mix of heat sources, it’s essential to ensure that the design optimises the performance of each technology. Good

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