Moving to smart prosumer buildings
As long as renewable power generation sources are weather-dependent, the UK will need to ensure sufficient back-up from other sources. Beata Blachut explains the importance of ‘prosumer’ buildings, ideally alongside 4G heat networks
Despite recent government announcements that the UK will soon be able to meet all of its electrical energy requirements from renewable sources, we can’t ignore the ongoing need for back-up sources when the wind isn’t blowing and the sun is obscured by cloud.
This assurance of security of supply is one of the three factors that make up the UK’s ‘energy trilemma’ – the other two being reduction of carbon emissions and cutting energy costs to tackle fuel poverty. All three of these need to be addressed in the UK’s energy strategy.
Why we need prosumer buildings
With so much of our renewable capacity controlled by the vagaries of the weather, we will always need this back-up capacity, irrespective of the installed renewable capacity. However, the UK’s current back-up capacity is mainly from fossil fuel power stations, mostly in remote locations a long way from the buildings that consume the generated power.
There are two key disadvantages to this arrangement. Firstly, when these inefficient power stations are operating they waste energy by discarding most of the heat generated. Secondly, their distance from the consumers of the electricity introduce distribution losses.
There are therefore several good reasons for decentralising a large proportion of the UK’s power generation capacity by installing more power generation in or near buildings – thereby creating ‘prosumer’ buildings that both produce and consume energy. Such systems also need the in-built ‘intelligence’ to take advantage of green electricity when it’s available.
This local power generation will ideally use combined heat and power (CHP) because, unlike gas-fired power stations, the heat produced during power generation can also be utilised within the building(s). Consequently, the electricity generated by CHP typically gives a 45% lower energy cost than that from gas-fired power stations, as well as a 28% lower carbon footprint.
Ideally, these buildings will also be connected to 4G heat networks, which are more complex and flexible than earlier heat network generations, so they are able to make optimum use of a wider range of heat sources.
One challenge with CHP is balancing the power and heat generation. Typically, CHP will produce 2kW of heat for each 1kW of electrical power. Where the CHP is connected only to the landlord’s distribution system in residential buildings, and where export of electricity is not viable due to low export tariffs, the response is often to turn off the CHP and use gas-fired boilers to meet heat loads, thereby negating the efficiency advantage of CHP.
An alternative for prosumer buildings is to continue operating the CHP and use surplus electrical power to run heat pumps to produce low grade heat. This can then be further heated using waste heat from the CHP to reach the required temperatures for domestic hot water (DHW) and space heating. Or the surplus power could supply an electric boiler to produce high grade heat for immediate use or storage.
In such prosumer scenarios, the emphasis is on maximising the run times of the CHP and avoiding the wasteful ‘dead time’ experienced at power stations.
How it might work
In most residential buildings the highest demand for power is in the mornings and late afternoon/evening. As noted above, when demand is low during the day, the current strategy is typically to turn off the CHP and use inefficient gas-fired boilers to meet any heating or DHW demand.
A smarter, prosumer approach is to replace one ‘consumer’ with another. So when the building’s occupants (consumer 1) stop demanding power, the heat pumps or electric boiler(s) switch on to provide a second consumer for the power. At all times, consumer 1 has first priority on power.
It is important not to oversize the heat pumps, because they are expensive and occupy a lot of space, so a combination of CHP, heat pumps and electric or gas boilers will often be the best solution.
Thus, in winter, rather than ‘topping up’ the CHP with gas boilers, low carbon heat from heat pumps can be used for pre-heating and then topping up with the CHP. In this scenario the CHP would be running for a prolonged time, maximising its efficiency advantages.
At other times of year the situation would be different. In spring and autumn the CHP may only run for half the time, whilst in summer it may only be run at peak times to meet DHW demands.
In situations with different demand patterns, some ‘smoothing out’ may be called for using heat storage.
It’s also important that these local solutions aren’t treated in isolation from the main power grid, especially as our use of wind energy increases. True ‘prosumer’ buildings will be able to react to the fluctuating availability of green electricity from the grid and turn off the CHP when appropriate.
For example, during stormy weather when power generation from wind turbines is high, the power available may exceed demand. Turning off a wind turbine is difficult and expensive, so a more efficient option is for buildings connected to the grid to quickly switch to this source of power. Indeed, in some countries (e.g. Denmark), the power companies will pay consumers to do just that.
Clearly if the CHP is switched off in favour of green grid electricity, some of this electricity must then be used to generate heat. This can be via the heat pumps or electric boilers referred to earlier, generating hot water for immediate use, or storing it for later use. Studies have shown that storing electrical energy as hot water is at least 100 times less costly than battery storage.
Regardless of how much wind and solar power capacity we install in the UK, there will always be a requirement – indeed, a responsibility – to ensure we have sufficient back-up capacity from power sources we can control (unlike the weather). There are therefore good reasons to take the pressure off the power stations and distribution grid by decentralising power generation to smart prosumer buildings integrated with 4G heat networks.
Beata Blachut is head of Strategic Business Development – Technical with SAV