Could solar-thermal be the solution for low-carbon heating?

Solar heat water heater and chimney

Well-established solar heat technologies could be playing a key role in creating a sustainable future energy system and meeting an increasing global demand for energy, according to our latest briefing paper. “Solar-thermal and hybrid photovoltaic-thermal systems for renewable heating” looks at the continuing potential of this technology and what stands in the way. Dr Alexander Mellor, Research Associate in the Department of Physics and co-author of the paper, takes a look at the practical steps required for the technology to take off.

Solar heating systems convert sunlight into useful thermal energy, which can heat homes and businesses, provide hot water and, in some cases, be used in industrial processes. Sunlight is the world’s most abundant energy source, and around half of the world’s energy demand is for heat. The need for solar heat couldn’t be clearer, so why isn’t it more abundant?

In recent years, there has been a shift in solar energy away from solar heat installations and towards solar electricity generated by solar photovoltaic (PV) panels, which has been brought on by the ever-falling production costs of PV solar panels. However, solar heat and solar electricity have different strengths, and should therefore be viewed as complimentary rather than competing technologies.

Solar heating panels have the advantage of generating much more energy per square metre – up to 4 times that of PV. It is also far cheaper to store heat than to store electricity. On the other hand, there are specific barriers to the growth of solar heat systems. Heat is far less versatile than electricity and cannot be easily transported over distance, which means that solar heating systems must typically be installed in homes and businesses rather than in large plants away from urban areas. What’s more, solar heating systems require maintenance, compared with PV, which requires almost none, and can be more intrusive than PV, as a storage tank is required in addition to the panels.

However, these barriers can be addressed by careful policy and by technical innovation.

Installing solar heat in new buildings

Integrating solar heating systems into the planning for new buildings is the cheapest and most effective way to roll out solar heat technology. Currently, government planning obligations stipulate fixed emissions standards for new builds, which encourages architects to integrate innovative technologies – like solar heat installations – into their plans. An advantage of this approach is that it is flexible – a range of different technologies can be used as long as the building meets the emissions standards. This means that strategies to reduce emissions compete on merit, driving competition and ensuring each building has the most appropriate solution for its needs.

However, as these innovations are eventually paid for by the building occupants, emissions standards do face opposition. From 2006 until 2015, emissions standards were increasingly stringent, with a view to requiring all homes to be ‘zero-carbon’ by 2016. However, in 2015, the government announced a sudden reduction in emissions standards, and removed the zero-carbon standard entirely: a move condemned as short sighted by building industry and environmental leaders. Reinstating the zero-carbon objective would certainly be a boon to the uptake of solar heat, and is probably one of the cheaper options for reducing emissions.

Introducing solar heat into existing buildings

80% of buildings that will exist in 2050 are already built. Therefore, retrofitting existing building stock is essential for solar heat to form a large part of the energy mix. Of course, there are numerous barriers to a large uptake of solar heat systems: they may not have the necessary capital to install one; and landlords may be reluctant to install and maintain such a system in their rental property. That’s why well-tuned government strategies and appropriate business models are essential.

For existing buildings, government incentive currently takes the form of the Renewable Heat Incentive: a subsidy from the taxpayer, which pays owners a fixed rate for each kilowatt hour they generate onsite from a renewable source (currently around 10 – 20 pence/kWh). These subsidies helped to establish a market for solar electricity. Of course, over-reliance on such subsidies is unsustainable and can hinder innovation. However, as the solar thermal market in the UK is still extremely small compared to our European neighbours, these subsidies could play an important role in helping to establish a market and supply chain for the technology – like they did with solar electricity.

The role of the supplier

There is some debate over the proper business model for suppliers of solar heating systems. Here, something can be learnt from the world of solar electricity, where companies like solarcentury increasingly offer to supply energy rather than just solar panels. Essentially, this means they guarantee a system will deliver a fixed amount of energy over a given period, and take responsibility for its maintenance. This allows customers to take on less risk, relieves them of performing maintenance themselves, and ensures the supplier takes proper care over the installation. In some cases, the company also offers finance and takes monthly payments. Translating these business models to solar heat could make it significantly more attractive to property owners and help to increase uptake of the technology.

Technical innovations to unlock the potential of solar heat

Technical innovations in solar heat are taking place over the country. These are aimed at reducing the cost of the panels, reducing the cost of thermal storage, using phase change materials to store heat without the need for large tanks, developing cooling systems that allow solar heat to be efficiently used for cooling, and developing hybrid photovoltaic-thermal (PV-T) collectors that produce both electricity and heat from the same panel. This portfolio of projects shows that the engineering and scientific community is taking a holistic approach to solar heat innovation, addressing barriers in generation, storage and use of solar heat.

These innovations will be increasingly important if we are serious about achieving a low-carbon energy system. For example, large parts of the population live in urban areas with limited roof space and limited space for extra thermal storage. As such, PV-T collectors will be vital for maximising the energy output of your available roof space, while phase change heat storage will take up relatively little space indoors. In addition, cooling (air-conditioning, refrigeration etc.) represents a large chunk of global energy use that is ever increasing. To be able to satisfy this demand using solar heat would be a huge step towards our climate goals.

Despite being a mature technology, solar thermal has not developed as large a market in the UK as it has done in other countries with comparable weather, such as Ireland, Germany and Denmark. With the right choice of government policy, business models, and technical innovation, the UK could yet become a leader in solar heat, as it has done previously in PV and wind technology.

Find out more

To hear more about climate change and the environment at Imperial College London, sign up to the Grantham Institute weekly update of news, views and events.

Download the Briefing Paper: Solar-thermal and hybrid photovoltaic-thermal systems for renewable heating

Leave a Reply