The collector – physical characteristics
The collector is the most important component of a solar thermal system but no particular make or type is best suited to every situation. A collector needs to fulfil the technological requirements of the operating, meteorological and climatic conditions of a specific project, as well as being appropriate in terms of design and architecture, economics and ecology.
In technical terms, there is a distinction between flat plate collectors, evacuated tube collectors, air collectors, uncovered collectors for swimming pool/low temperature heating, and concentrating collectors (such as parabolic trough collectors, compound parabolic concentrators (CPCs) and Fresnel collectors).
However, the basic structure and main components of a collector are always the same. Figure 1 shows an example of a flat plate collector.
In the centre of every collector is an absorber, which converts solar radiation into heat as effectively as possible and transfers it to a circulating fluid (a mixture of about 60% water and 40% polypropylene glycol; or air in air collectors) with minimal loss of heat. Absorbers with selective coatings are nearly always used in flat plate and evacuated tube collectors for hot water and heating applications in Central and Northern Europe. The properties of these surfaces provide very high absorption (typically α = 0.92 to 0.96) in the wavelength range of solar radiation.
The absorber gets warm while operational, so it is also important to have low emissivity (typically ? = 0.05 to 0.1) in the wavelength range of infrared radiation. This way the absorber's heat loss due to infrared radiation can be drastically reduced and the collectors are much more efficient with an overall heat loss coefficient of around 4 W/(m2 K).
A non-selective black paint has var epsilon-values around 0.9 which would give a heat loss coefficient of typically 6-8 W/(m2 K) for an otherwise identical panel. Collectors with non-selective absorbers can nevertheless be used for domestic hot water in simple thermosyphonic systems in hot climates.
The transparent cover of a collector should ideally have the highest possible transmission in the entire wavelength range of solar radiation. Special solar glass is used (white glass or low-iron glass) with a typical transmittance of τ = 0.89 to 0.91. By using anti-reflective coatings the transmittance can be increased to between 0.94 and 0.96. The glass is toughened because it needs a higher mechanical and thermal resistance than window glass. Structured glazing is used in many modern flat plate collectors but this is mainly for aesthetic reasons.
The back can be insulated using many different materials including mineral wool and polyurethane foam (only suitable for sufficiently low temperatures). In addition outgassing, reaction to moisture, temperature and fire resistance, workability in the manufacturing process, expected lifetime and (depending on the housing construction) mechanical properties, also play a role.
New concepts under development include vacuum-insulation panels, filling the collector with inert gas, complete evacuation, and insulation using microfoam products based on polyisocyanate (which is more resistant to high temperatures).
In order for the collector to fulfil its expected service life of 25 years, the housing must also meet many tough requirements:
It must be rainproof, mechanically stable enough for storms, snow and hail, and must enable good roof and facade integration. The collector mounting structure, which can be considered part of the housing, is particularly exposed to wind and snow loads.
The frame can be made of a variety of different materials including wood, aluminium, steel and polymers, and may also serve to regulate the inner climate of the collector. It is important to control air humidity in a collector under all operating conditions, as this has a major influence on reliability and service life.
When assessing panels available on the market it is important to take into account not only the price/performance ratio but also durability, reliability and ease of installation. Reliable operation and long service life are essential for all regenerative systems.
As a way of promoting high overall standards and to make sure that public funds are spent well, most European countries now ask for Solar Keymark product certification as a prerequisite for granting subsidies to the end-user. Collectors must fulfil a number of important requirements in accordance with the EN12975 standard to qualify for this quality label:
- Internal pressure of the absorber;
- High temperature resistance;
- Exposure to weather conditions;
- Fast external temperature change;
- Fast internal temperature change;
- Penetrating rain;
- Mechanical load;
- Freeze resistance (optional);
- Impact resistance (optional); and
- Final inspection.
The ongoing European project QAIST (Quality Assurance in Solar Heating and Cooling Technology), funded by the Intelligent Energy Europe programme and led by the European Solar Thermal Industry Federation (ESTIF), also has a strong focus on quality issues. The project aims to enhance the competitiveness of the European solar thermal industry and increase consumer confidence through improved standards and certification schemes; harmonisation in testing and certification; and wide dissemination of quality concepts throughout Europe.
Global harmonisation of collector standards and certification is also on the QAIST agenda. In this area its work is closely connected to the new International Energy Agency's (IEA) Task 43 project (part of the Solar Heating and Cooling Programme) which focuses on solar rating and certification. In terms of collectors, this aims to make the EN 12975 standard better adapted to new developments such as tracking and concentrating collectors. This will make development work more efficient and speed up the introduction of new products into the market.
Performance characteristics of a solar thermal collector
The thermal performance of solar collectors, measured according to the European collector test standard EN12975, can be characterised by the following parameters:
- Peak power curve and efficiency curve (parameters η0, a1 and a2);
- Incidence Angle Modifier (IAM); and
- Thermal capacity of the collector (Ceff).
The peak power curve of a flat plate collector refers to module power produced under an irradiation value of 1000 W/m2 (taken as a function of the temperature difference between the mean fluid temperature and ambient air temperature). The same parameters can be used to specify the efficiency of a collector.
When comparing the output of flat plate and vacuum tube collectors, it is important to specify in terms of power generated by either ‘the module’ or ‘gross area’. It is a common misunderstanding that vacuum tube collectors are much more efficient per unit area than flat plate collectors, even in the low to medium temperature range used for domestic water heating; this is not the case. Furthermore, basing a comparison on gross area – the minimum area required on the roof – shows the ‘true’ output from the collector.
High-performing flat plate collectors can provide strong competition to vacuum tube collectors in single-stage cooling processes, which is important to take into account for designers of solar cooling installations.
The incidence angle modifier for a solar collector is also important. It indicates the factor by which the performance changes when a collector is not illuminated perpendicular to the front surface. The IAM has a significant influence on the performance of untracked collectors, for which the direction of incident irradiation changes continuously during the day and throughout the year.
Future developments in solar thermal collectors
In Central Europe about 90% of solar collectors sold are flat plate, and around 10% are vacuum tube collectors. Approximately 95% of installations are used for tap water heating and heating support.
The forthcoming developments in this sector will focus on improving the price/performance ratio while maintaining the current level of quality, reliability and durability. This is particularly important when new materials are used to reduce costs.
A major challenge for the solar industry remains how to improve the integration of solar collectors and solar photovoltaic (PV) panels into buildings, making energy generation a key feature of construction components like walls, roofs and windows. To provide the building sector with ready-to-use systems, some window-integrated collectors are entering the market. In this way collectors are built to absorb and use solar energy, and at the same time they can be used as a shading device for additional temperature control. They can also be used as an interesting design pattern within the facade.
In addition a number of ‘new’ collector technologies – products that have not yet reached a significant market share – are under development. These include air collectors (pictured) and collectors able to use condensation heat from the ambient air humidity (which are beginning to appear in some combined solar/heat pump systems). The latter combination is getting a strong focus in Europe and several new collector and system concepts can be expected as a result. The new Task 44 project within the IEA's Solar Heating and Cooling Programme is devoted to combinations of solar collectors and heat pumps, and the QAIST project is dealing with issues of quality assurance.
New technologies are often induced by niche markets, which have their own requirements and can cope with innovative ideas. An air collector system, for example, may be suitable where there is a need to raise the solar fraction within a building's heating sector, but where the roof area is limited. Also, the technology of air-based collectors has the potential to use simpler system configurations, and possibly even cheaper materials, to support a heating system.
When building large collector fields – for example in a facade – stagnation and flow patterns are a big issue for water-based collectors. This is not the case for air-based systems. Heating and cooling distribution systems within bigger buildings, as well as dehumidification units, are often based on air and this offers some potential for cost-effective integration. One disadvantage however is that air has a much lower heat capacity, leading to high flow rates and high energy demand for running a system. The use and configuration of systems as well as the characterisation of air collectors are all subjects of ongoing research projects.
Current development work on high temperature concentrating collectors is also very important. Concentrating collectors such as parabolic trough collectors and Fresnel collectors can only use direct solar radiation and must therefore be used in areas with a high fraction of direct irradiance. Adding the concentration of solar radiation they can achieve efficiencies of about 40-50% – at temperatures between 150 and 250°C. They are therefore suitable for new applications such as thermal power stations, two-stage thermal driven cooling processes and high temperature heating systems for industrial processes.
In Fresnel collectors, concentrated solar radiation is reflected onto a fixed receiver mounted above the reflector surface. The reflector consists of individually tracked reflector strips.
Renewed interest is also being shown in the development of PVT collectors, which consist of a solar cell integrated in a thermal collector absorber. In these collectors, the waste heat that arises in the solar cell (only about 15% of solar radiation is converted into electricity) can become useful.
New technologies can be assessed for performance and quality using the same provisions as those mentioned above. However, in addition to the QAIST project, established test standards need to be appropriate for measuring performance characterisation and evaluation as well as for ensuring reliability and durability, so that these technologies can develop and have a strong entry into the marketplace.
About the authors:
- Professor Matthias Rommel is from the SPF Solartechnk Institute, Hochschule Rapperswil
- Peter Kovács is from the SP Technical Research Institute of Sweden
- Korbinian Kramer is based at the Fraunhofer-Institut für Solare Energiesysteme ISE.
Renewable Energy Focus
Volume 11, Issue 5, September-October 2010, Pages 36-38