In presenting a research agenda, the solar thermal sector sets out a strategy to reach a 50% contribution to Europe's space and water heating requirements by 2050. David Appleyard summarizes the document, which reveals what must be achieved if this ambitious goal is to become reality.
The research efforts and infrastructure needed to supply 50% of the energy for space and water heating and cooling across Europe using solar thermal energy has been set out under the aegis of the European Solar Thermal Technology Platform (ESTTP). Published in late December 2008, more than 100 experts developed the Strategic Research Agenda (SRA), which includes a deployment roadmap showing the non-technological framework conditions that will enable this ambitious goal to be reached by 2050.
A strategy for achieving a vision of widespread low-temperature solar thermal installations was first explored by ESTTP in 2006, but since then the SRA has identified key areas for rapid growth. These focus points include
the development of active solar buildings, active solar renovation, solar heat for industrial processes and solar heat for district heating and cooling. Meanwhile, amongst the main research challenges is the development of compact long-term efficient heat storage technology. Once available, they would make it possible to store heat from the summer for use in winter in a cost-effective way.
The ESTTP's main objective is to create the right conditions in order to fully exploit solar thermal's potential for heating and cooling in Europe and worldwide.
As a first step for the development of the deployment roadmap and of the Strategic Research Agenda, ESTTP developed a vision for solar thermal in 2030. Its key elements are to establish the Active Solar Building – covering 100% of their heating and cooling demand with solar energy – as a standard for new buildings by 2030; establish the Active Solar Renovation as a standard for the refurbishment of existing buildings by 2030 (Active Solar renovated buildings cover at least 50% of their heating and cooling demand with solar thermal energy); supply a substantial share of the industrial process heat demand up to 250°C, including heating and cooling, desalination and water treatment; and achieve broad use of solar energy in district heating and cooling.
'The benefits of increased solar thermal energy usage are immense', explained ESTTP chairman Gerhard Stryi-Hipp, adding: 'Supporting R&D into the next generation of solar thermal applications must have a high priority for governments everywhere in Europe, because solar thermal is a key to reaching Europe's goal of 20% renewable energy by 2020.'
Market drivers
Heating accounts for a significant proportion of the world's total energy demand with the building sector alone consuming 35.3%, of which 75% is for space heating and domestic water heating (IEA, 2006). In Europe, the final energy demand for heating and cooling at 49% is higher than for both electricity at 20%, or transport at 31%.
Despite these figures, for a long time low-temperature solar thermal only played a minor role compared to other renewable energy sources. It was mainly considered suitable for water heating needs and consequently, in future energy strategy scenarios, renewable heat generation frequently played only a small role.
However, the situation has changed dramatically. Without doubt, the European goal of covering 20% of energy needs with renewable energy can only be reached with a significant increase in the renewable heating sector. Within this sector, it is low-temperature solar thermal technology that has the greatest potential.
The large technological development potential of low-temperature solar thermal has been triggered not only with enhancements to system types and components, but primarily in the development of new uses for the technology, such as solar heating, process-heating generation, district heating and solar assisted cooling. Volatility in crude oil and natural gas prices, along with increasing import dependency, have further increased public attention and interest.
It is expected that the energy and climate crisis will drastically change the heating market over the next two decades. In new buildings, a tightening of energy performance requirements, including obligatory use of renewables, will be increasingly required. In the existing building stock, energy savings will become the key driver for renovations, and district heat operators will become more interested in, and possibly be forced to, increase the share of renewables.
For industrial process heat and cooling, the key driver will be the need to reduce growing energy costs, and possibly the cost of emission allowances at the carbon market.
All these developments will lead to a sharp increase in the use of solar thermal, and the subsequent need for new technologies.
Active solar buildings
The ESTTP vision is to establish the Active Solar Building as a standard for new buildings by 2030. For existing buildings, the aim of the ESTTP is to foster Solar Active Renovation. The aim is also to cover substantially more than 50% of the remaining heating and/or cooling demands with active solar energy.
There are already many Active Solar Buildings with a proven track record in Central Europe. The first one-family house covering 100% of its heating requirements with solar energy was created in Switzerland back in 1989. More recently, the first multi-family buildings with 100% solar thermal coverage were introduced.
Cost-effective and practical solutions to heat storage represent a key technological challenge, since the widespread deployment of Active Solar Buildings largely depends upon it. The ESTTP vision assumes that, by 2030, heat storage systems will be available with an energy density eight times higher than water.
For solar collectors, significant improvements are still possible, particularly in terms of cost reductions and design. However, low temperature collectors, which are usually used on buildings, are already very efficient.
High energy efficiency values can be reached through high insulation standards, which reduce losses, and optimal architecture which integrates passive solar measures, such as active windows, shading or ventilation systems.
Furthermore, the productivity of solar thermal systems is enhanced by heating and cooling systems that require a low temperature difference between the supply system and the indoor temperature, such as radiant surfaces, floor heating and cooling, ceiling heating and cooling, and heating/cooling of ventilation air. Most of these solutions already exist, but there is still the potential for cost reductions, increased performance and easier integration.
The demand for cooling in buildings is growing dramatically; and not only in Southern Europe. Despite the impressive growth rates, solar-assisted cooling is still in the very early stages of development. Over the next decade, the first systems supplying domestic hot water, space heating and cooling for buildings will be installed.
However, significant R&D must be carried out in order to exploit the potential for further technological development, which will pave the way for the large-scale deployment of solar cooling.
In the future, solar active systems, such as thermal collectors, PV-panels and solar hybrid systems, will be obvious components of roofing and facades and they will be integrated into the construction process at the earliest stages of planning. Walls may also function as a component, supporting thermal energy storage through the application of, for example, phase change materials. One central control system will lead to an optimal regulation of the whole heating, ventilation and air conditioning (HVAC) system, maximizing the use of solar energy. Heat and cold storage systems will play an increasingly important role in maximising solar thermal contributions.
While a very small number of Solar Active Buildings have already been showcased, making this a mainstream building standard by 2030 will only be possible if significant technological progress is achieved in high-efficiency solar collectors that will increase the energy gained under winter conditions, while maintaining high levels of durability and increasing the cost efficiency of the manufacturing and installation process.
Other key developments include compact, time indifferent thermal storage technologies that significantly reduce the space required for heat storage devices. This will lead to cheaper and more practical seasonal heat storage. Improved solar thermally driven cooling systems will make it possible to cover much of the rising demand for air conditioning with solar energy, while intelligent control systems of the overall energy flows in buildings will contribute to a reduction in energy consumption and the optimization of solar energy usage.
Industrial heating and cooling
Solar Heating for Industrial Processes (SHIP) is currently at the very early stages of development. Less than 100 operating solar thermal systems for process heat are reported worldwide, with a total capacity of about 24 MWth. Most of these systems are of an experimental nature, and are relatively small-scale. However, there is great potential for market and technological developments, as 28% of the overall energy demand in the EU27 countries originates in the industrial sector, and much of this is for heat of below 250°C.
In the short term, SHIP will mainly be used for low temperature processes, ranging from 20°C to 100°C. With technological development, more and more medium temperature applications, of up to 250°C, will become market feasible. Around 30% of the total industrial heat demand is required at temperatures below 100°C, which could theoretically be met with SHIP using current technologies, and 57% of this demand is required at temperatures below 400°C, which could largely be supplied by solar in the foreseeable future. In several specific industry sectors, such as food, wine and beverages, transport equipment, machinery, textiles, pulp and paper, the share of heat demand at low and medium temperatures (below 250°C) is around 60%. Tapping into this potential would provide a significant solar contribution to industrial energy requirements.
Substantial potential for solar thermal systems exists in the food and beverages, textile and chemical industries, as well as in washing processes. Among the industrial processes, desalination and water treatment (such as sterilization) are particularly promising applications for the use of solar thermal energy, as these processes require large amounts of medium-temperature heat, and are often necessary in areas with high solar radiation and conventional energy costs.
Clearly, the use of solar heating for industrial processes should be part of a comprehensive approach, which also takes into account: energy efficiency measures; the integration of waste heat into processes; and a reduction in heating and cooling demand through the use of a heat exchange network.
An ample choice of solar thermal collectors is commercially available for low temperatures (operating temperatures up to around 80°C–90°C) and for high temperatures (>250°C, mainly used for electricity generation). The development of cost-effective and reliable medium-temperature collectors, which can meet the requirements of most industrial processes, is now required.
Other components of solar systems also need to be adapted to this range of temperatures. For example, development of the industrial solar market would benefit from the development of a new generation of compact and/or seasonal heat storage systems, and from advanced controllers.
Furthermore, despite the fact that many processes in the industry operate at temperatures below 100°C, the heat supply of most industrial machines is currently provided by steam networks operating at between 140°C and 180°C. This makes the use of solar thermal less attractive, or even impossible. Switching to lower temperatures would imply significant investment on infrastructure and network modification and process redesign, which reduces the attractiveness of solar energy. Nonetheless, new technologies can be developed, which allow processes to operate at lower temperature. One example is the reduction of bath temperatures in pickling plants. In some cases, processes can also be efficiently redesigned to make them more compatible with the daily and/or seasonal cycle of solar energy supply. Moreover, when new, long-term industrial process facilities are planned, there is always the possibility of subsequent solar add-ons.
Integrating solar thermal into industrial processes will be a complex process, requiring support from energy agencies and other public players, dedicated to specific industrial sectors. Research is necessary in a number of fields, including stagnation behaviour and management of large collector fields; monitoring; and system optimization methodologies.
Another requirement is the need for dedicated design guidelines and tools. Currently only a few engineering offices and research institutes have experience with SHIP installations. Planning guidelines and tools for typical industrial uses need to be made available to a wider community of experienced engineers. This would mean that other potential users could be offered a solar solution, system design costs would fall, and the broader experience would increase the effectiveness of such installations.
However, solar systems are capital intensive, as costs are mainly up front, and industrial companies often optimize their processes with short-term return on investment expectations that cannot currently be met by solar systems. The wide market development of industrial and process solar would also require dedicated financing and contracting solutions, the lack of which is currently an important barrier to growth. It is crucial, therefore, to rapidly create a market, in order to reach the minimal critical mass required to start benefiting from economies of scale. While R&D can increase potential and reduce costs in the medium term, financial incentives and widespread public-funded demonstration projects are an absolute necessity.
District heating and cooling
Currently, around 9% of the total heating needs in Europe are covered by block and district heating systems. This share is much higher in a number of countries, especially Eastern Europe and Scandinavia.
Within district heating systems, solar thermal energy can be produced on a large scale and with particularly low specific costs, even at high latitudes, such as in Sweden and Denmark. However, only a very minor share (less than 1%) of the solar thermal market in Europe is linked to district heating systems, which together account for less than 0.5% of EU installed solar thermal capacity. However, their combined capacity is still higher than that of 25,000 small solar domestic hot water systems.
The prevalence of Scandinavian countries is surprising, since solar radiation is lower in this region. Central and Eastern European countries and district heating systems in Southern Europe offer much better conditions.
Typical operating temperatures range from low (30°C) to high (around 100°C) for water storage. The majority of plants are designed to cover the heat load over the summer months (hot water and heat distribution losses) using diurnal water storages. However, some are equipped with seasonal storages and cover a larger part of the load. The seasonal storages comprise water in insulated tanks, the ground itself, aquifers and a combination of ground and water. More than 80% of Europe's existing plants are equipped with flat-plate collectors, mostly large module collector designs. Most plants also have pressurized collector systems with an anti-freeze mixture – usually glycol and water – while a few plants in the Netherlands have drain-back collector systems.
Several solar district heating systems, especially in Sweden and Denmark, have ground-mounted collector arrays. This can be a very cheap solution, when surfaces are available and solar is connected to a network serving existing buildings.
In the short-term, the broader use of solar energy within district heating (and cooling) systems is mainly a question of policy – namely, incentives, regulation, and the demonstration of existing technologies. In the medium- and long-term, considerable R&D efforts are needed to utilize the full potential of large-scale solar systems linked to district heating. The need for basic and applied research is mainly related to the development of durable and cost-effective (plastic) liners and water resistant insulation materials for long-term (seasonal) storage. Basic and applied research is also required to further develop large-scale solar collectors, as well as dedicated control devices and optimization strategies.
Widespread deployment of solar thermal
Compared to other continents, Europe has the most sophisticated market for different solar thermal applications, with a relatively wide mix of different applications such as hot water preparation, space heating of single- and multi-family homes and hotels, large-scale plants for district heating as well as a several pilot systems for air conditioning, cooling and industrial applications. However, also in Europe, the majority of the new solar thermal systems are installed on residential homes for heating domestic hot water only, with solar typically providing 40%–80% of demand. Nevertheless, there is already a clear tendency towards combined systems for hot water and space heating in countries like Germany and Austria, where 50% or more of the newly installed systems are combined systems.
Additionally, in markets like Spain, France and Austria, large systems for multi-family homes have a significant share. The systematic development of the market for collective systems is important to reach the short to medium-term goals, since the majority of the European population lives in such dwellings.
Of course, deployment must go hand-in-hand with substantial improvements in the energy efficiency of buildings and of heat consuming processes. It is imperative that both pathways develop as rapidly as possible to dramatically increase efficiency and to replace the remaining heating and cooling demand with renewables.
Higher efficiency values create the necessary conditions for a fully renewable supply of thermal energy demand, freeing scarce fossil fuel resources for other purposes where they are less easily replaceable.
While oil and gas prices may have dropped in the current downturn, using fossil fuels or electricity for heating and cooling buildings is likely become too expensive for most people in the longer term and will be seen as an unacceptable squandering of resources.
By overcoming a series of technological barriers, it will be possible to achieve a broad-scale market introduction of advanced solar thermal applications at competitive costs.
ESTTP predicts that with political support mechanisms and technical developments based on increased R&D, realistic growth rates of 20% in the solar thermal market are achievable. These growth rates would lead to an installed capacity of 970 GWth by 2030 in the EU, supplying about 8% of the total heating demand.
Combined energy conservation measures and increased efficiency in buildings that could slice some 40% of total heat demand would enable solar thermal systems to supply about 20% of the overall heat demand in the EU-27 by 2030.
The long-term potential of solar thermal is to provide about 50% of EU heat demand by 2050, an installed capacity of 2576 GWth or 8 m2 per inhabitant.
The strategy concludes that low-temperature solar thermal must play an important role in the research programmes of the EU and its member states. The funding for solar thermal research must be significantly increased and the research capacities must be systematically expanded.
'Solar thermal can provide much more than just domestic hot water', says ESTTP chairman Gerhard Stryi-Hipp, adding: 'Already today solar thermal systems combining hot water preparation and support to space heating are in wide-spread use in Central and Northern Europe. But to reach our goal of 50% of heating to be supplied by solar thermal energy, new applications have to be developed and deployed.'
David Appleyard is associate editor of Renewable Energy World.
e-mail: rew@pennwell.com
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