Often described as the sleeping giant of renewable energy, solar heating technologies have been a woefully overlooked option to massively increase the renewable contribution to energy supply. However, with superb efficiency and a wide range of applications, the technologies that make up the solar thermal sector are indeed making inroads to the market.
With a surface temperature of some 6000°C, the colossal fusion reactor that is our sun radiates truly prodigious amounts of energy. Indeed, so much power is emitted that, residing some 93 million miles away, the upper atmosphere of the Earth continuously receives an average of around 1.4 kW/m².
After passing through the atmosphere, the light reaching the surface of the Earth is mostly split between the visible and infrared spectrum, but of the energy which remains, the atmosphere, oceans and land masses absorb approximately 3850 ZJ (x1021) per annum. By way of perspective, total global energy consumption is currently estimated at around 500 EJ (x1018).
It is therefore somewhat surprising that use of direct solar radiation has not focused more closely on solar thermal technology. In terms of investment, significantly larger sums worldwide are ploughed into solar PV technology. This is partly due to the relative investment requirements necessary to initiate production. The comparatively simple technologies and materials required in solar thermal systems have allowed small and medium enterprises to dominate the sector. Conversely, the high investment costs associated with solar PV manufacturing have largely precluded smaller players from entering the market and left the field dominated by major industrial operations such as Sharp or BP.
Nonetheless, the relative levels of R&D and manufacturing investment between PV and solar thermal become even more surprising when considered in terms of their relative energy efficiencies. Experimental PV systems are currently yielding maximum efficiencies of over 30%, while the most efficient commercially available solar thermal technologies are yielding efficiencies of approximately 70% under optimum conditions for load, positioning, temperature and such like. It is no surprise that solar thermal is frequently referred to as ‘the sleeping giant of renewables’ by its proponents.
Solar thermal principals
A number of technologies exist for the extraction of thermal energy from light, but all rely on the same physical principals. The absorption characteristics of a material are of course determined by the frequency of light reaching the surface. An ideal solar absorption surface is one with minimal reflectance, with high solar absorption characteristics across a wide range of frequencies, and with a low thermal emittance.
Typically, solar absorbers are made from either copper or aluminium sheets or plates — materials with a good thermal conductivity — with an efficient absorber coating, such as a matt black paint, or more sophisticated ‘selective’ absorber coatings.
According to the US National Renewable Energy Laboratory (NREL), selective absorber surface coatings can be categorized into six distinct types: intrinsic; semiconductor-metal tandems; multilayer absorbers; multi-dielectric composite coatings; textured surfaces; and, selectively solar-transmitting coating on a blackbody-like absorber.
Intrinsic absorbers use a material having natural properties that result in the desired spectral selectivity. Semiconductor-metal tandems absorb short wavelength radiation and have low thermal emittance. Multilayer absorbers use multiple reflections between layers to absorb light and can be tailored. Metal-dielectric composites, or cermets, consist of fine metal particles in a dielectric or ceramic host material. Textured surfaces have a high solar absorbance by way of multiple reflections among needle-like, dendritic, or porous microstructure. Selectively solar-transmitting coatings, meanwhile, are typically used in low-temperature applications.
By exploiting these absorption characteristics, solar energy is collected by a working fluid — frequently water but also molten salts or air — and the heat is used in a wide variety of both direct and indirect applications.
Applications for solar heating and cooling
From the passive direct heating of swimming pools and domestic hot water, space heating and cooling, through district heating and industrial process steam, and on to multi-MW-scale power generation installations, the abundance of potential solar thermal energy is seemingly matched only by the variety of applications for which it is suitable.
Inevitably, larger systems are more appropriate for applications in which the working temperature is more than 80oC or when there is a large year-round thermal demand. Hotels, nursing homes and hospitals are typical high volume applications which can benefit from the use of larger thermal solar installations. Higher temperatures also enable the use of adsorption chillers and thus cooling applications.
However, although district and block heating applications for solar thermal have the major advantage of lower specific costs where district heating networks already exist and are not taken into consideration, the largest source of demand for the technology remains the domestic sector.
With the potential to provide an average northern hemisphere household with up to 70% of its domestic hot water needs, solar thermal systems can make a significant difference to domestic greenhouse gas (GHG) emissions. For example, households account for nearly 25% of UK GHG emissions, of which half is expended for space and water heating. In central and northern Europe it has become relatively common to install solar thermal systems that provide heat for both space and water.
Commercial and industrial buildings often use the same types of technologies that are seen in the residential sector, but may also employ technologies that, because of their size or performance characteristics, would prove impractical or too costly in a domestic setting. A good example comes from the solar air heating that requires the type of facade typically seen on a commercial building such as a distribution warehouse. However, process heating, and even cooling using absorption chillers, are also good examples of industrial and commercial applications of solar thermal technology.
Solar thermal company Paradigma, for instance, are marketing a commercial-scale vacuum tube type system in which pure water, rather than an antifreeze solution, is used. This approach allows the working fluid to be used directly in process applications without the complications associated with potential chemical interactions between the steam and process or heat exchanger systems. The technology was launched as the Aqua System — with more than 25,000 systems installed so far according to Paradigma — the new commercial-scale offering was launched in early 2007. The company has now installed something over 180 systems.
In each case, supplying space heating or cooling, process steam and such like through the use of solar thermal technology saves both expenditure and fossil-fuel alternatives.
The simplest type of solar collectors are typically found on outdoor swimming pools and consist of a transparent or black plastic or rubber sheet through which the pool water is heated passively. The energy is stored in the pool water itself. More sophisticated systems circulate the pool water through the sheet for improved absorption characteristics.
Vacuum tube collectors have more than 95% market share in China, which is by far the largest solar thermal market, and therefore worldwide the most commonly found systems are vacuum or evacuated tube type.
Normally, an array of tubes are connected together to form a collector field and within each vacuum tube runs a central absorber surface attached to a pipe carrying the working fluid. On the inside, a reflector may also be found, maximizing radiation falling onto the absorber surface.
As with the domestic vacuum flask, the evacuated tube which surrounds the absorber surface minimizes thermal emissions — a tube also significantly reduces re-emission of energy. Indeed, with an internal temperature that may reach more than 200oC in an overheat or stagnation situation, the tube may be cool to the touch. Such systems are consequently more efficient than simpler designs and tend to operate at higher temperatures.
Due to the effectiveness of the insulating layer, evacuated tubes can also be more efficient in colder conditions, but they are also more expensive to manufacture and more fragile. Furthermore, product quality issues can arise.
An alternative is the flat-plate collector. In its most crude manifestation, the flat plate collector consists of an insulated box with an absorber surface, say black painted copper or aluminium. Behind this absorber sheet run copper tubes which carry the working fluid, typically either water or an antifreeze solution. These tubes are welded to the back of the absorber surface either ultrasonically — in the case of copper absorber and copper circulation tubes — or using laser welding technology where, say, an aluminium absorber is matched with copper circulation pipes. Often, a transparent glass cover over the absorber surface improves the thermal characteristics of the flat plate collector by creating a greenhouse effect that increases the temperature of the absorber surface well above ambient. This glass may ‘solar’ (or low-iron) glass which has a high transmissivity allowing more solar irradiation to pass through.
A novel hybrid design of both flat plate and evacuated tube has been developed by one manufacturer and recently launched by Genersys. Working with partners ThermoSolar, Genersys says its systems combine the aesthetic appeal of a flat plate, glazed, solar panel with the insulative qualities of vacuum collectors. In addition, when the vacuum is lost on the new evacuated flat plates, re-evacuation is easily achieved, the company says.
Another design of solar thermal system uses air as the working fluid. Sometimes known as unglazed transpired collectors (UTC), these systems can raise the air temperature by more than 20°C and deliver outlet temperatures of 45°-60°C.
Typically such systems use an absorber facade, likely to be nothing more sophisticated than galvanized steel sheeting painted a dark colour, and set — in the northern hemisphere – on the south-facing surface of a large building. As sunlight falls on the building, energy is absorbed, which is then transferred to ambient air drawn into the building through a matrix of small airholes in the absorber. An air gap between the perforated steel sheet and the structure of the building allows the heated air to be sucked in through the holes and out into the ventilation system.
This heated air may be used directly — employed for space heating or drying, or to provide pre-heating for air or process requirements for instance. It may also be used indirectly, for example heating water.
Such developments are ideally suited to new build projects. For example, Latvia will soon be home to the largest UTC systems in Europe with some 2100 m² of solar panels, delivering more than 1 MW of thermal energy to a new warehousing complex.
Developed by Conserval Engineering, the SolarWall installation will be used for heating and ventilation purposes when completed by the end of summer.
The short payback period of 3-12 years for such systems can also make them a cost-effective alternative to flat plate or vacuum tube technology.
Storage and control
In all types of solar thermal collector system, after the working fluid has passed through the absorber it is either used directly or transferred to some type of insulated storage medium or thermal reservoir. Under normal operations, if the absorber is found to be more than a few degrees warmer than the storage tank, circulating the working fluid will yield usable energy.
Unlike electrical energy, thermal energy is relatively easy to store and a range of solutions have been explored. These technologies use a secondary material or fluid, such as water, steam, concrete, graphite, molten salts, and phase change materials, which absorb the collected energy through a heat exchanger. Commercial-scale projects have advanced this technology sufficiently to be able to generate electricity throughout the hours of darkness.
In a typical domestic system, water from the collector is passed through a heat exchanger, often within the well insulated storage tank itself, to heat water which is stored until required.
For example, in domestic systems which use an antifreeze solution in the absorber circuit, usually by adding a small amount of glycol propylene, the water which is actually used does not enter the collector at all. This type of arrangement, so-called fully-filled, is typically found in northern Europe or the US where sub-zero temperatures could see the working fluid freeze — damaging the absorber. Fully-filled systems also usually have expansion vessels to absorb steam in an overheat scenario.
An alternative solution to the use of antifreeze and expansion tanks are so-called drain back systems. When a set temperature is reached, the pump turns itself off and the water in the system drains back into the tank. However, while this does stop the pipes from freezing, it also means that the collector cannot provide hot water in very cold conditions.
By far the most common solar water heating systems are passive circulation systems, so-called solar siphons or thermosiphons. Dominating the market in countries such as China, Israel, Turkey, Cyprus, and Greece, such systems rely on gravity and the process of convection to circulate the working fluid as it is heated.
An alternative is to actively circulate the working fluid with a electric pump, which uses a small amount (less than 10% of the total energy produced) of electricity.
Even so, despite the relative simplicity of many aspects of solar thermal technology, controls for such systems are generally more sophisticated than may be found on an equivalent conventional installation. The thermal protection systems seen in drain-back systems are one example. However, perhaps more significant is the differential thermostat that determines when the circulation pump will be activated in pump-circulated systems. In addition to the operational and safety function requirements of the solar thermal system, such controllers may also operate a back-up on-demand heating system that can be used when solar output is low.
Room for growth
There is no dispute that solar thermal has a vast potential to improve the renewable energy contribution of many nations, but this goal remains some distance away. Part of the issue is the widespread perception that solar thermal technologies are only suitable for those regions with high-intensity insolation. This is a misconception. For instance, as Kevin Brennan, head of sustainability for Velux, says: ‘Currently only 0.004% of the UK’s housing stock has solar water heating, yet over 76% of homes in this country could successfully make use of this technology.’ Brennan continues: ‘While installing solar thermal in all homes across the UK could prove a challenging task, a commitment from housebuilders to incorporate this technology into all of their new builds could still have a significant impact on reducing the UK’s carbon emissions. Within 30 years, the new homes being built today by housebuilders will be approximately 30% of the entire housing stock, so even small gains today will be significant gains in 30 years time.’
Even so, while it may not have achieved even close to the penetration level its efficiency, simplicity and cost-effectiveness may warrant, there is cause for optimism.
A major boost for the industry is the projected development of new, cheaper, materials. While rising prices in copper have seen a number of manufacturers switching to cheaper and lighter aluminium as an absorber surface, the use of alternatives such as polymers is also attracting a great deal of interest.
Although current generation plastics tend to become brittle under high levels of sun exposure and also tend to suffer in an overheat scenario, there has been progress and such developments are expected to dramatically reduce the cost of solar thermal systems in the next few years.
In addition, a number of large buildings material and equipment companies such as Vaillant, and Buderus, part of Bosch Thermotechnology Ltd, have introduced solar thermal packages of late. There is growing interest among developers too, with a number of recent acquisitions by larger renewables players, now ready to offer a full range of technologies. For example, Renewable Energy Systems Ltd (RES) announced the acquisition of solar thermal company Future Heating Ltd this year.
Policy drivers related to climate change are also having an impact, with the forthcoming EU Renewable Energy Directive raising the profile of solar thermal among commercial, industrial and commercial operations.
Perhaps most fundamental to the growth of solar thermal installation, the issues of energy security of supply and the spiralling costs of fossil fuels are certain to improve the economics of alternatives.
However, it should also be remembered that solar heating and cooling technology is nothing new. In the 1870s, French solar pioneer Auguste Mouchout demonstrated its potential by making ice using a solar steam engine attached to a refrigeration compressor. Despite this remarkable success, his project was abandoned shortly thereafter as falling coal prices rendered it uneconomic.
Times have certainly changed, and once again it seems that the conditions are right for solar thermal to come out from the shade.
David Appleyard is Associate Editor of Renewable Energy World.