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Hybrid solar thermal-heat pump on trial

David Appleyard

With space and water heating still accounting for close to half of Europe’s primary energy demand, innovation in heat networks offers considerable scope for environmental and economic benefits. Now a hybrid demonstration project is being commissioned that features the UK’s largest solar thermal array for district heating, coupled with both heat pump and storage technology.

In the relatively sunny South West of England the UK’s largest solar thermal district heating system is on trial at the heart of a major housing and commercial development zone.

Although still being fully commissioned, the installation is already supplying renewable heating and hot water to some 1500 homes. But what makes this 1,814 m2 solar thermal array stand out is - coupled with a heat pump and energy storage - it forms part of a hybrid system. 

A demonstration project aiming to show if such a system is technically feasible and if the technologies offer the potential for further development, at its heart is a bid to maximise the usable capacity available from the solar array.

When required, say on overcast days or during the ‘shoulder’ seasons of spring and autumn, the heat pump can be used to raise the temperature of the solar array output from the range of 50?C to the 85?C plus required to supply the district heating network.

Furthermore, the use of buffering storage capacity allows the use of the water source heat pump to be optimised, while enabling heat energy to be delivered on demand, or produced during low-electricity cost periods. By using night-time electricity to boost the day time sunshine to deliver early morning heating, this project is a huge thermal battery.

Project development

Backed by the former Department of Energy and Climate change – now rolled into the new Department for Business, Energy & Industrial Strategy – the solar thermal-heat pump-storage hybrid emerged from a demonstration competition looking for innovation in heat networks. With an overall programme supported to the tune of £300 million (US$490 million) over five years, innovation in heat networks offers system savings of £6 billion (US$7.8 billion) to 2050, according to UK government figures.

For E.ON’s Project Sunshine at its Energy Centre project in Cranbrook, not far from Exeter, DECC capital funding of £1.4 million (US$1.8 million) supported the asset purchases.

The existing energy centre which comprises of gas boilers and combined heat and power (CHP) plants will be supplemented by these new assets, eventually supplying 3,500 new homes, as well as 30,000 m2 of office, commercial and schools space and 130,000 m2 of industrial space at the neighbouring Skypark industrial zone. 

Once fully developed, the low density housing will be supplied with a network stretching some 80 km – 38 km of pipe has currently been laid to domestic consumers and a further 2.5 km to commercial consumers, carrying water at more than 70?C and 1.8 bar. 

Commenting on the selection of the site Dr Nilton Chan, Head of Technical Delivery at E.ON, says: “At Cranbrook we have more than 1500 customers already connected for heat which is currently supplied by the gas boilers and CHP engines in our Energy Centre, so the demand is there and we also have the space for a solar array.” 

Still, he emphasises that the installation is only a demonstration project, not least because the land on which the solar thermal array stands is only leased until required by the on-going development of the nearby Skypark. “Although this may not provide a complete, finished solution right away, we are using it to discover the optimal arrangements and controls which could be put into use elsewhere,” says, referring to the array.

The Cranbrook hybrid energy system

The array itself, which was supplied and installed by SK Solar / ARCON Demark, delivers just above 1.2 MWth at peak. It stands some 800 metres from the energy centre, where the storage and heat pump are located.

The Cranbrook Energy Centre also currently contains a 550 kWe, 685 kWth gas CHP unit, as well as five gas boilers with a total combined capacity of 12 MWth.

Within the adjacent bay, designed to become a bioenergy fuel depot as the development progresses in scale and greater heat generation is needed, stand the two 50 m3 thermal stores, with a total storage capacity of 2,042 kWth, and an 850 kW heat pump. 

Cranbrook also has a 250 kWp solar photovoltaic (PV) array installed on the roof, which is capable of powering the heat pump. 

Known as a ‘Neatpump’ and manufactured by Star Renewable Energy, the unit was shipped from Glasgow to Cranbrook in the autumn of 2015. 

Indeed, with the DECC contract only in place by mid-June 2015 and solar deliveries not commencing until the October, at times conditions on the flood plain site bought works to a standstill. Nonetheless, around six months behind schedule, commissioning began in March 2016, with final commissioning of the heat pump element on-going.

Optimising hybrid operations

Comprising flat-plate fixed-axis solar receivers with glycol-water as a heat transfer fluid, in operation heat is transferred via a plate heat exchanger to one of two thermal stores, either high or low temperature. If the solar array is performing strongly, and a high temperature is achieved this is directed to the high temperature 85?C store or for distribution to the DH network. 

If panel performance is down, the fluid may either be directed back through the array for additional heating or directed to the low temperature 50?C thermal store. If desired, the heat pump may then be employed to lower the temperature of the fluid in the low-temperature store, transferring the heat to the operational requirements of the high temperature store.

An 85?C supply temperature in winter is moderated to 75?C during summer, whilst the return feed, at perhaps 35?C, is directed to the low-temp store. During low-performance days, the capacity for storage of low temperature heat enables the use of the heat pump to be optimised.

Metering points at five locations measure the heat, flow rate, and flow and return temperatures in the solar system. 

Explains Chan: “The individual technologies are known and trusted but it is the integration into our system and the control philosophy to continue delivering energy effectively that needs to be worked through.”

He continues: “I think one challenge we're still working on is the control - operating the full integration of the system - the solar thermal and the heat pump working together. How can we make the system optimal almost by intelligence of control systems, depending on the weather, or indeed prediction of tomorrow’s weather?”

Designing the right heat pump

The Cranbrook heat pump uses ammonia as its working fluid, using a single stage circuit.  

First commercially developed and used in the 1850s, Dave Pearson, Director of Star Renewable Energy, explains that a number of technical developments now enable heat pumps to operate efficiently at a range of temperatures.

“A heat pump needs to be durable and it needs to have higher energy efficiency. One of the ways we get higher energy efficiency is using the right working fluids.” 

For instance, although HCFCs and related compounds are relatively easy to work with, due to their properties they also consume more energy in operation. However, while switching to alternatives, like ammonia, brings efficiency and environmental sustainability benefits, it also represents significant technical challenges.

Says Pearson: “We are operating up at 90?C, and that is round about 52 bar. That was basically the root cause of all our challenges, and in fact was the biggest reason that people hadn't tried to do it, or they didn't think it was possible. Everything from simple things like valves and oil filters to pumps, compressors, heat exchangers as well to a certain extent, all developed problems associated with the use of ammonia at these much, much higher pressures. Over 5 years at Drammen in Norway we have resolved issues one by one. The heatpump delivers over 85% of the annual heat in the Drammen facility, the largest of it’s kind or temperature anywhere in the world. ”

He also emphasises the role of proving grounds such as the Cranbrook installation: “It's a very good example of a high temperature heat pump. This proves that an industrial heat pump can scavenge waste heat from higher temperatures. So it proves high temperature on the delivery end, and it proves high temperature or medium temperature in the source end, that is really significant going forward.”

Technical and commercial challenges

To fully commission the system conditions must reach a ‘Goldilocks’ state, not too hot nor too cold. However, with the ever-reliable British weather such conditions have, at the time of writing, not quite been met. Chan explains: “It takes time to commission the heat pump. Remember heat pump is still quite new technology in a district heating context, so that's one thing. The other thing is, we need the weather to support it. For the heat pump, commissioning requires an almost constant flow of 50 degrees’ temperature.“

Nonetheless, initial life cycle assessment shows operational CO2 emission savings in the range of 55-117 tonnes per year. These savings will improve to 137– 166 tonnes per year as grid emission intensity falls, according to the UK’s University of Exeter, which is conducting technical advisory and life-cycle assessment work on the project.

“Only 60% of the solar array has been commissioned and since the start of commissioning on the 19th April, we have collected 130 MWh of free solar heat and of which circa 100 MWh was delivered to our existing DH customers,” says Chan.

However, commercial challenges remain if solar thermal-heat pump hybrids are to make a significant impact on heat network infrastructure.

 “One of the key things for E.ON was to see whether the market will require some kind of incentive scheme from the government if such as system is to replace CHP as a heat source for district heating. One of the outcomes potentially is that we will be recommending that this is a potential future solution for district heating networks,” Chan says.

Chan suggests such systems may require incentives of a similar order to offshore wind, but he says: “We are still looking at the data because we don’t know exactly how much sun we can capture. Cranbrook is in the South West, so it’s relatively sunny. Weather patterns will need to be factored in when looking at similar projects in other parts of the country that might not be so sunny.”

One key conclusion has already emerged though: “If you can have a bigger thermal store, then you can save your peak generation. That means you can store quite a lot of energy in the thermal store, and then release it when it’s needed. It would give us first of all, longer running hours for the heat pump, and then secondly, we can store more energy, just in case the following day there is no sun at all.”

This is a point picked up by Pearson: “Heat pumps bring an interesting twist: we can effectively create devices that are thermal converters, but also bring an element of time change into it and at a fraction of the cost of electrical storage and without using rare-earth metals.”

Cranbrook is a proof of concept exercise, but potentially demonstrates new operating models in which renewables, heat pumps and even seasonal heat storage make flexible renewable heating a reality.

The final report on the Cranbrook trial is due towards the end of March 2017.


David Appleyard is a freelance journalist specialising in energy and technology.

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Energy efficiency  •  Solar electricity  •  Solar heating and cooling