As the European Union strives to achieve its 2020 renewable energy targets, solar heating and cooling technologies could potentially play an important role, not least because of their adaptability - solar heating technologies are compatible with almost all conventional heat back-up sources and have the potential to be widely applied. But, solar heating and cooling technologies have another string to their bow, in that there is a strong local economic argument in their favour. Since a significant portion of the value chain cannot be delocalised, and some of the technologies - specifically flat-plate or vacuum tube collectors - are relatively simple to produce, these technologies have significant potential for local manufacture, with the associated benefits for local economic development both in Europe and in the developing world. Meanwhile, research has shown that this technology will be most efficient, benefitting from economies of scale, when combined into larger district heating systems.
A recent JRC report on district heating and cooling potentials1 notes that building heat load in Europe is not likely to shrink significantly due to the difficulty and cost of retro-insulating buildings. The report also notes that there are practical difficulties with the supply of renewable electricity to the current building stock in cities due to the cost of upgrading distribution networks. In these conditions, local heating and cooling technologies may provide an opportunity to increase the share of renewables in the energy mix in communities across the EU. The report underlines that the integration of district heating based on combined heat and power (CHP) could pave the way for the development of a robust infrastructure supporting the integration of wind power and solar energy, helping to deal with the issue of intermittency and the need to meet peak heat demands.
With respect to solar heating and cooling technology, there are two distinct types available: active and passive. Passive technologies aim to maximise the absorption of heat by buildings in cold weather, while minimizing heat intake in the summer. Active systems, on the other hand, allow for more efficient use of the captured solar heat. Temperature levels in these systems can vary from as low as 25° C to as high as 1,000° C in concentrating solar technology, with a corresponding range in possible applications - from swimming pool heating to power production from which waste heat can be used for other thermal applications.
The level of development of solar heating and cooling technology varies depending on the application. Some technologies used in areas such as domestic water heating are relatively mature and can be competitive, and solar-assisted district heating and low-temperature industrial applications are close to commercialisation in some European countries, while other applications, such as solar space cooling and heating require further targeted R&D to ensure cost effectiveness and widespread uptake.
In common with other renewable technologies, solar heating and cooling technologies reduce exposure and sensitivity to energy price fluctuations, as most costs occur at the initial investment stage and operating costs are minimal. Solar cooling also benefits from the correlation between supply and demand - demand for cooling is highest when the solar resource is at its strongest. Moreover, systems are local and therefore more efficient, with reduced need for transmission. Solar heating and cooling systems can also be used to complement other RES technologies and energy efficiency measures, with possible synergies with CHP and biomass systems for the provision of hot water and process heat; and with photovoltaic systems and CHP for power generation. The JRC report explores these synergies, specifically the use of large-scale solar water heating as a source to supply district heating systems. In its Solar Heating and Cooling Technology Roadmap2, the International Energy Agency (IEA) also notes that, in economic terms, larger systems are generally more effective due to economies of scale, but there has nevertheless been a significant increase in demand for smaller residential systems recently, especially in Spain.
A Danish study quoted in the JRC report reaches the same conclusion - finding that the cost of solar energy rapidly decreases with size, indicating that solar-based district heat is much more economic than individual systems. Moreover, the cost of storing heat becomes considerably cheaper as the size of the store increases, which indicates that using solar heat distributed by district heating systems will be more economic than individual roof mounted units. In the study, district heating is seen as fundamental to energy saving in Europe, as it is perceived as being the ultimately flexible tool for integrating power station waste heat, industrial waste heat, large-scale solar heat in summer, and surplus wind energy via electrode boilers and heat pumps. Moreover, the use of multi-tasking units that provide space heating, cooling and hot water from one appliance maximises the solar fraction and results in improved environmental outcomes and added end-user benefits. In addition, the latest technological developments mean that solar heating can be stored from summer to winter in underground pit storage facilities, which basically use landfill technology combined with a floating insulated cover.
The International Energy Agency (IEA) forecasts the development and deployment of solar heating and cooling by 2050 to produce 16.5 exajoules3 (EJ) of solar heating and 1.5 EJ of solar cooling, or over 16% of energy use for low-temperature heat and nearly 17% of total energy use for cooling by that time. The annual collector yield of all water-based solar thermal systems in operation by the end of 2010 in 55 countries surveyed for an IEA report was 162,125 GWh. This corresponds to energy savings equivalent to 17.3 million tons of oil and 53.1 million tons of CO2.4. To foster a wider uptake of solar heating and cooling technologies, the IEA recommends the setting of medium-term targets for mature technologies and long-term targets for advanced technologies, accompanied by differentiated economic incentives such as feed-in-tariffs for commercial heat and subsidies or tax incentives for end-user technologies. From the demand side, barriers hindering uptake include a general lack of information about solar heating and cooling technology, which can be dealt with by organizing awareness campaigns.
At a time when NIMBY opposition is stalling many renewable energy projects, such as onshore wind farm projects in various countries in the EU, the fact that solar heating and cooling technology has the potential to make a significant contribution to local economies, combined with the relatively unobtrusive nature of the technology - it has low visual impact and no other controversial side effects, such as noise, odour or landscape pollution - may provide a significant boost to its wider adoption, and the integration of this technology into large district heating and cooling systems will allow these systems to benefit from economies of scale and become competitive as a result.
For more information:
1JRC Scientific and Policy Reports, Background report on EU-27 District Heating and Cooling Potentials, Barriers, Best Practice and Measures of Promotion, EUR 25289, 2012.
2Technology Roadmap, Solar Heating and Cooling ©OECD/IEA, 2012.
31 exajoule = 1018 joules.
4Solar Heat Worldwide, Markets and Contribution to the Energy Supply 2010 (2012 Edition) Weiss, Werner and Mauthner, Franz IEA Solar Heating & Cooling Programme, May 2012.