Solar thermal energy

Under Solar thermal refers to the conversion of solar energy into usable thermal energy.

• 5.1 solar tower power plant
• 5.2: Wind power plant
• 5.3 parabolic trough power plant
• 5.4 Paraboloidkraftwerk ( Dish )
• 5.5 solar pond power plants

• 6.1 Overview
• 6.2 Methods
• 6.3 Application
• 6.4 Advantages of direct heat use

Introduction

The incident on the earth's surface radiation power worldwide is a daily average (based on 24 hours), about 165 W / m² ( with significant variations depending on the latitude, altitude and weather). To this source of energy to help yourself, man has begun to look more closely at the issues solar thermal ( heat from solar radiation ) and photovoltaic ( electricity from the sunlight).

The entire incident on the earth's surface energy amount is more than ten thousand times greater than the energy requirements of mankind, the potential is greater than that of all other renewable energies together.

History

The first applications of solar thermal utilization go to antiquity ( 800 BC - 600 AD ) back, were used as fuel or concave mirror for focusing of light rays. The passive use of solar thermal energy has been practiced by the architecture of its buildings in ancient Egypt, Mesopotamia, and in the early South American civilizations. Here doors were positioned such that they were at high noon on the side facing away from sun for example. In cold climates, windows and doors are preferentially oriented on the leeward side of the house, but possible in the direction of the midday sun.

Documented are the architectural considerations of the Roman architect Vitruvius. He wrote: " In the north, it seems, the building must be equipped with a flat domed ceiling as possible closed and not open, but are applied directed to the warm climes there."

In the 18th century naturalist Horace - Bénédict de Saussure invented the forerunner of today's solar panels. Triggered by the first oil crisis in the mid -1970s were developed viable concepts for the use of solar energy.

The world's first patent for a solar plant was awarded in 1891 to the metal manufacturers Clarence M. Kemp of Baltimore. It was a simple heat collector for hot water.

The Olympic torch was lit and is traditionally since ancient times on burning mirrors.

Theoretical foundations

The sun generated by the running in their interior nuclear fusion power of about watts and delivers it in the form of radiation. From the spherical solar surface this radiation power is delivered approximately equally in all directions. Strictly speaking, the radiation output of the sun varies both spatially and temporally, see for example, solar activity and space weather for more information. The earth orbits (actually in an elliptical orbit ) at an average distance of about 150 million kilometers around the sun, this distance is also called astronomical unit. In view of the radiation power the Earth moves on a spherical surface around the Sun with a radius of one astronomical unit. The radiation power is distributed on this spherical surface is on average

,

Which is called the solar constant. The value of the solar constant is variable, in that it is misleading to speak of a constant, the term but has naturalized. The adoption of the solar constant allows simple calculations and estimations. While in the near-Earth space a vertically oriented to the sun area actually receives the instructions given by the solar constant value, the Earth's atmosphere causes a noticeable attenuation. Through reflection and absorption by clouds, aerosols and gases, this value is considerably reduced by the atmosphere. Because depending on the latitude, the radiation must travel a longer path through the atmosphere ( the absorbance is proportional to the path length and density of the medium air, thus proportional to the mass of air which must traverse the solar radiation on the way to Earth's surface ), is calculated depending upon abode on earth a different radiation power. From a clear midday sky in summer (higher sun = shorter path through the atmosphere ), the sun shines in Germany only with a maximum in winter even with. There are weather stations with solar meter that will also allow the daily fluctuations observed. Thus, the solar radiation received at the earth's surface increases from sunrise until noon as in a sine function, and then drops in the same way again. At night, the solar radiation is, of course, averaged over an entire year, resulting in about Germany

Integrated over time results from the received solar radiation, the usable solar energy. The radiated energy per year in Germany is about

.

This corresponds to about half of the theoretically achievable value, which is mainly due to cloud cover. The Federal Statistical Office expects solar thermal collectors, which are used for space heating, a year with an average yield of

Or.

This value is again much lower, since it takes into account other factors that affect the yield in practice also.

The selective conversion of the incident in the form of electromagnetic waves solar radiation into thermal energy can in principle be done in two ways. On the one hand by means of solar collectors which are based on the principle of absorption. Secondly, by concentrators ( concave mirror or a plurality of planar mirrors) that are based on the principle of reflection. The internal or concave mirror, or a plurality of individual mirrors, sun tracked then focus the sunlight, whereby an increased light intensity on an absorber, and thus a higher temperature can be achieved in the heat transfer medium.

In diffuse irradiation thermal utilization of the global radiation is usually not possible.

Solar panels

To a large extent solar panels are made ​​in the form of so-called roof systems. But there are also exceptions.

Types of solar panels

We distinguish the following types:

→ Main article: Solar Panel

Flat-plate collector

Parabolic trough collector

Solar thermal collectors without the concentration of radiation to raise the temperature

• Flat plate collectors operate at an average temperature of about 80 ° C. In them the light is not bundled, but directly heats a flat heat-absorbing surface that conducts heat well and is crisscrossed with tubes in which the heat transfer medium is. In these panels, a water-propylene glycol mixture is mostly ( 60:40 ) was used as heat transfer medium. The addition of 40 percent propylene glycol antifreeze to -23 ° C and below freezing without increase in volume is achieved (to avoid a possible frost damage ), and a boiling point, 150 ° C and can be more depending on the pressure. There are now newer flat panels, which are fitted in place of the insulation material with a vacuum insulation (similar to vacuum tube collectors ). This increases the efficiency by reducing energy losses. The useful annual thermal energy, which provides a non-vacuum insulated flat plate collector, is about 350 kWh / m².

• Evacuated tube collectors consist of two concentric glass tubes built. Between these glass tubes there is a vacuum that allows the transfer of radiant energy of light to the absorber, but greatly reduces heat loss. In the inner tube is a heat transfer medium, usually a water-glycol mixture, which is heated and driven by pumping the heat transported. There are also so-called " open systems ", heat the water directly. These collectors typically operate up to an operating temperature of approximately 150 ° C. Evacuated tube collectors are more efficient than flat plate collectors, but they are typically expensive to purchase.

Solar thermal collectors with the concentration of radiation to raise the temperature

• Vacuum tube collectors can also contain reflectors ( see picture above) which concentrate the radiation on the pipe with the heat transfer medium. There are also on the market densely packed vacuum tube without reflector. The concentration effect, however, depending on the version and is strongly on the one hand cause less light seems unused between the vacuum-insulated heat transfer pipes through the roof tiles
• On the other hand, they allow to arrange the vacuum absorber with a larger clearance, which saves costs, and by the concentration of the radiation to the vacuum absorber, the temperature in the heat transfer medium rises faster, and is increased, whereby the minimum system temperature, and thus the timing of the usability of the energy achieved faster and the system is longer and better use of energy due to the higher temperature.

• Parabolic trough collectors use the focusing of the light rays on a centrally extending absorbing heat conduction. It should be noted here is the significantly higher working temperature lying between 200 and 500 ° C. Therefore, oils are used as the heat transfer medium.

• Solar towers, in which individual flat mirrors track the sun so that the light is concentrated at the top of a tower on the actual absorber. By this method, very high temperatures of over 1000 ° C. can be produced. The theoretical limit in this case is the radiation temperature of the Sun from about 5500 ° C. As the heat transfer medium is air, oils, or liquid sodium are used.

The aim is thus targeted absorbing all possible incident on the collector of solar energy.

Problems are the reflections of solar radiation in the absence of anti-reflective coating dar. This means that only a part of the solar radiation reaches the absorber. Newer collectors sit in some cases a non-reflecting special glasses which reduce the mirrored and thus not usable radiation.

Higher temperatures that exceed the intended operating temperature, can possibly lead to thermal cracking of antifreeze and thus damage and permanent inoperability of the collector. Achieving such temperatures is avoided by an appropriate technical design of the collector itself and by integrating an appropriately powerful circulation pump.

The ratio of the recovered heat energy of the incoming radiation on the collector is the energy efficiency. This is at the current state of research for home applications between 60 and 75 %.

Areas of application in everyday life

Solar thermal energy is used in the private sector primarily in the context of building heating and air conditioning. In conjunction with a good thermal insulation and passive use of solar radiation, the need for additional heating energy already reduced greatly. A thoughtful passive use of solar energy can also go to Central Europe to such an extent that an active heating system is no longer required. The most typical examples of a passive use of solar radiation are the greenhouse and conservatory. Roof overhangs oversized double glazed south-facing windows can have a cooling effect in summer, and in winter use ( by then the lower position of the sun ), the light passing through the window heat radiation to space heating. A similar effect can be achieved by absorbing panels in which the sunlight behind a transparent damping material impinges on a black absorber surface and the underlying wall -heated. These passive techniques are used in the so-called solar architecture. Since modern office building ( for example, the Commerzbank Tower in Frankfurt am Main and the Post Tower in Bonn) today often have an almost fully glazed facade, resulting in an excess of summer solar heat. Here special glasses can help, which block the thermal radiation of the high- noon sun in the summer, but are transparent to lower beams, as they occur in winter and in summer outside of lunch hours. Such special glasses can also be controlled selectively electrically. Often a ranging over several storeys Atrium with fountain is available to obtain a cooling natural thermals.

As part of building the classification based collector solar thermal systems results as "active " technology due to the use of active, ie mostly electrically powered circulators within the heat cycle. However, even a passive use is conceivable, such as rooftop installations in frost-free climates, the function according to the passive thermosiphon principle, or even when operated on the same principle collectors in small systems, such as for heating the water for outdoor showers.

Collectors can be used for water heating, as an independent and full heating, or to support an existing heating otherwise. Any other additional heating is required only in old buildings, where either the insulation is not sufficient, or is present in relation to the volume of space to small roof size than that of the heat demand could be fully covered by collectors. Another reason can also be an unsuitable roof orientation a permanent shade of the building, or (for a pitched roof ). Optimal is an alignment of the panels to the south, where in the roof mounting regional differences need to be considered, so that the system is at any time of the day in the shade.

If these points are taken into account, otherwise a heater is always completely replaceable. This is mainly due to better environmental and operating characteristics of solar thermal energy over other forms of heating, such as the various forms of cogeneration pellet heating systems is desirable. Solar thermal and passive solar building techniques require less maintenance prone, in this case, and have the non-existent fuel demand significant advantages. For the plant operator all the running costs omitted ( low to on electricity costs of about 8 € per year for the operation of the electric circulation pump at 7 W power and 5300 running hours per year, based on an average single-family house and a green electricity price of 21.5 cents per kWh ). Total Socially produce the equipment or construction methods for the same reason, no additional traffic and do not compete with agriculture for valuable arable land, which on the basis of renewable raw materials would be the case with the widespread use of fuel burning cogeneration plants. They also do not produce any emissions of particulate matter still manually to remove residual ash.

The storage of predominantly gained in our latitudes in summer heat energy over long periods is ensured by thermochemical heat storage, in which the heat with virtually no loss is chemically bonded and released again delayed seasonal, due to buffer heat storage, as water, or by latent heat storage, eg. based on paraffin, in which a large part of the heat energy in the phase transition from solid to liquid is stored, and partly because its an over simple water tanks by a factor of 1.5 results in higher heat capacity. The use of seasonal storage on the basis of water or passive thermal mass has the disadvantage of a much larger space required to achieve the same heat capacity. However, this disadvantage can be with skillful design of new buildings are unnecessary (eg by thermal activation of the area enclosed by the foundation soil mass ), so that just passive thermal mass or water may be desirable solutions for heat storage for cost reasons.

By make sun rays on the collector, they give depending on the absorption capacity of the collector typically 60-75 % of their energy to the heat transfer liquid. It is then pumped by a pump into the heat exchanger of the memory. A to a temperature sensor connected controller (also called solar controller ) uses the circulation pump in motion as soon as the temperature of the heat transfer fluid in the collector exceeds a certain threshold. The control unit settings affect the efficiency of the overall system, and depend on the power consumption of the circulation pump and the pump power. When using a buffer memory allows the control set so that the pump is running when the temperature of the heat transfer fluid is above the temperature of the lower ( tend coolest ) water buffer in memory. There it transfers its heat to the colder waters of the buffer memory. The buffer water warms up by increases in memory up, and then can be used via the heat exchanger for heating of separate circuits for drinking and heating water, the water cycle for showering, washing and the like can be used more, and ultimately in the home use the full need for heated " process water " covers.

Modern washing machines and dishwashers some have separate hot water connections, what solar thermal building can also contribute by providing already heated drinking water to improve efficiency in electricity consumption of household appliances.

The buffer memory traditionally fulfilled the function of a time-delayed heat absorption and release. When using a thermochemical storage season, which also promptly and as required heat can be extracted, the use of a buffer is no longer necessary. The seasonal storage can completely take over the function of the buffer memory ( delayed heat absorption and release ). Any latency when starting the extraction of heat can be absorbed by a very small-sized internal buffer memory which is integrated into the seasonal storage. However, this can also be avoided by a correspondingly low-latency design of the device itself from the outset. The heat transfer fluid is fed directly to the chemical ligation of the thermal energy of the season storage, and heating side takes over the storage season the task of an instantaneous water heater. This eliminates the separate buffer memory, which reduces the cost, and ongoing heat loss avoids the constantly incurred in buffers (despite insulation) as opposed to the long term almost loss-free thermochemical heat storage systems.

Orientation

Solar thermal power plants are normally equipped with flexible tracking technology, so that the question of orientation does not arise. In parabolic trough power plants mostly single-axis tracking systems are used in Paraboloidkraftwerken and other horizontal -tracking concentrator designs typically biaxial. With statically mounted collector systems in the building, however, an optimal placement angle is essential in order to ensure a high yield. We distinguish between vertical tilt angle and, depending on the hemisphere, south and north deviation ( azimuth angle)

In Europe, let the best returns at a collector slope of 30 to 45 degrees and a direct southern exposure achieve (azimuth 0 °). In this case, a small Südabweichung of up to 20 ° are not taken into account. Because of following relationships also a deviation even can be beneficial in some cases: In the morning hours the humidity is usually higher, which has a stronger haze of air result. Furthermore, bearing in mind that the maximum air temperatures are usually reached in from 13:00 clock to 15:00 clock. Due to the higher ambient temperature increases the efficiency in many installations. This suggests possibly for a south-south -west orientation. On the other hand, it may increasingly come in the afternoon to cloud formation in some areas. This counteracts, or speaks at otherwise constant environmental conditions for an orientation to the south-south -east.

The optimum mounting angle should be calculated on an individual basis based on regional weather data. For this offer various manufacturers to appropriate simulation software.

The installation angle of collector systems, which are intended primarily for heating, are traditionally often optimized to a maximum income during the winter months, instead of a maximum annual total return. This is done with the intention, without the involvement of a seasonal storage predominantly in winter heating demand incurred directly in time to completely cover, with the smallest possible and cost optimized design of the collector. Here, the panels are set up with about 40-45 °, thus significantly steeper than for example in grid-connected photovoltaic systems would be the case, which are optimized with 30-35 ° to the highest possible total annual income towards. By the procedure to adjust the yield curve at the time distribution of the heating energy demand between summer and winter months for simple systems without seasonal storage is reached, and is thus available for the same collector in winter more heating energy directly available, in exchange for an even greater yield loss in the summer, but the more you take into account due to the then very low energy requirements for heating. Accordingly, for heating systems using the heat energy requirement is obtained mainly during the summer and transition times ( as in the heating of outdoor showers ) will be the optimization of the radiation during the transitional periods makes sense because the sunlight during the consumption period transition period - summer transition time during the transitional periods on is lowest.

With the integration of a seasonal memory in the system, however, a static assembly with steeper mounting angle from the viewpoint of a favorable cost / benefit ratio is counter-productive, since the information given by the memory possibility of delayed heat generation and sharing the use of the highest solar radiation in the summer time offset during the winter allows and decisive is not the problem of the highest heating demand in winter for the design of the plant. Instead, in the summer by a factor of two to three higher solar radiation to only slightly higher by a steeper yield mounting angle predominates in winter.

On the other hand is ideal (regardless of the use of a seasonal memory ) the flexible tracking of the collector so that the solar radiation in summer and in winter is used optimally. On flat roofs, a two-axis tracking be installed on pitched roofs, however, is in any case only one (mostly single-stage ) Adjust the vertical inclination angle possible. In practice, such solutions are generally used rarely in buildings due to the maintenance requirements of mechanics - often outweighs the advantage of easier handling and lower cost statically mounted panels. Also tracked collectors are not suitable for roof installation, which is used especially in new buildings with pitched roof often.

From an Angle of inclination greater than 45 ° is not recommended in general, as this decreases the yield. The importance of Aufstellwinkels in the winter months > or <45 ° anyway playing a minor role because the solar yield dense cloud cover is much more through the in Central Europe in the winter months is limited. A shallow mounting angle can, however, result in the summer to thermal surpluses with the risk of plant shutdowns. Ideal is a mounting angle of 90 ° in relation to the sun's altitude on 21 March and 23 September.

The argument to avoid hail damage can possibly speak more on mounting angle than 45 °, if it is likely in the region with correspondingly dangerous hail.

Energy payback time

The energy payback time of a solar thermal system is 12 to 24 months, ie At this time the panels are applied to the same amount of energy of the heater, which had to be spent for the production, etc. of the plant. The lifetime of the panels is at least 30 years.

Economy

Goods solar thermal systems in the 80s and 90s in Central Europe computationally hardly be replicated and were mostly used by ecologically interested, so is today (2013 ) a solar system at oil prices of 90 cents per liter and the heat energy generated from 0.10 € / kWh most economically useful calculable. The payback period of a solar thermal system depends not only on the sunlight mainly on the durability of the components, since the system expects only the saved fuel energy. With the use of solar heat for heating support south exposure, high-quality solar collectors and a good memory layers, in addition to a hydraulically balanced sensible heating is essential. Low flow temperatures increase the solar gain enormously.

One of the first German collectors Bauer still has plants have been running since the early 80s.

This flat-plate collectors with respect to the vacuum tube collectors are largely maintenance free.

In contrast to photovoltaic German manufacturer of solar thermal systems are successfully in world markets and benefit especially from the enormous growth of the solar heat generation in China.

Promotion

Part 1: Research and Development

In order to support the market of the emerging renewable energy technology in their own country, the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety ( BMU short ) established extensive promotion measures. In the newly developing global market for solar thermal power plants German companies have excellent opportunities due to the evolving with BMU - promotion technologies. The first power plants in Nevada and Spain were realized with significant German participation. In 2007 alone, a funding of 5.9 million euros has been re-approved by the BMU, in addition to the support of another 5.9 million euros, which flow into ongoing projects. As of 2008, the promotion of renewable energies in the heat market continues with new priorities. For the so-called market incentive program, a total of up to 350 million euros. This is much higher than in previous years. As part of the Integrated Energy and Climate Programme of the Federal Government, the BMU has adopted a new funding policy for the market incentive program that will indefinitely from 2008 onwards. From 2009, up to 500 million euros available for the program. The budget increase is done from the proceeds through the auctioning of emission allowances. A central goal of promoting according to these guidelines is to strengthen investment incentives the sale of renewable energy technologies in the heating market, thereby helping to reduce their costs and improve their profitability. From the year 2008, as a result, set special incentives for market development with the newly introduced innovation funding for new or particularly innovative technologies in accordance with these guidelines.

Part 2: Current use

Generally, the use of solar thermal systems is funded by the State, depending on size and application. The BMU is designed with the promotion in the field of low- temperature solar thermal energy from it, to increase the share of solar thermal energy in heating and cooling significantly. You should increasingly contribute to the substitution of fossil fuels and thus to reduce CO2 emissions. The plan is to achieve a tenfold increase in the installed thermal solar power by the year 2020. To realize this, the funding measure has been called " Solarthermie2000plus " to life. It is aimed at owners correspondingly large existing or newly constructed buildings or buildings Direction for the integration of solar thermal systems in the blueprint.

Background: The energy policy in Germany is aimed equally at

• The profitability for producers and consumers,
• The conservation of the environment and resources, and in particular the reduction of CO2 emissions,
• The security of energy supply.

Part 3: Specific numbers (as BAFA 3/2013 )

Solar collectors for combined water heating and heating support

• For an initial installation BAFA promotes ( Federal Office of Economics and Export Control ) to 40 m² each installed ( or part thereof ) m² with 90 euros for combined-cycle plants. Pure drinking water systems will not be supported in one-and two -family house.
• During the initial installation of solar collector systems of more than 20 m² to 100 m² gross collector area in an apartment building with at least 3 units and in large non-residential buildings for combined water heating and heating support, a promotion of 180 euros per m² gross collector may be requested.
• Process heat is supported with up to 50% of net investment ..

In addition, bonus incentives are for the following measures:

• Boiler replacement Bonus: The bonus of 500 euros will be granted if the same is installed for installation of a solar collector system for combined water heating and heating support an existing boiler without condensing technology against a boiler with condensing boiler (oil and gas).
• Regenerative combination Bonus: Simultaneous establishment of an eligible biomass facility or an assisted heat pump system. The bonus is 500 euros.
• Efficiency Bonus: Plus 50 % to the base funding for the establishment of an eligible solar collector system for combined water heating and heating support or eligible biomass facility in a particularly well-insulated building. Insulated to be particularly well include a building, in which the 2009 calculated according to EnEV allowable transmission heat loss HT below ' by at least 30 %.
• Bonus for highly efficient solar pumps (per pump maximum 50 euros ).

Solar thermal power plants

→ Main article: Solar thermal power plant

In solar thermal power plants, solar energy is focused by mirror systems on an absorber and the heat generated there by using conventional technology ( eg steam ) used for electricity production. Depending on the nature of the focusing mirror system trough power plants, power towers and dish systems can be distinguished.

Solar tower power plant

To an approximately 50 to 150 meters high tower a field of heliostats ( mirrors internal ) is located, the computer- controlled track the sun and the reflected beams combine on one attached to the top of the tower absorber ( "Receiver "). Through this flows a heat medium can be heated by the concentrated solar energy up to 1000 ° C. A heat exchanger system, the heat energy produced is used to generate steam which drives a turbine coupled to a generator, as in the power plant systems are already known, and thus generates electric power.

Currently, various technological approaches based on different heat transfer media (air, water or steam or molten salt ) and receivers exist ( shell and tube heat exchangers, atmospheric or pressure- charged volumetric structures) build.

This allows relatively high power densities of ~ 37 Watt / m² mirror surface and about 25 watts / m² of floor space at the site realize Spain. Based on a global radiation of 126.18 watts / m² in Germany instead of prevailing at the site in Spain global radiation of 205 W / m² would therefore be expected only with a yield of ~ 17 W / m² floor area in Germany.

Currently the largest solar tower power plant " Solar Two " is in the Mojave Desert in California / USA and delivers an output of about 10 MW. In Germany, a test and demonstration facility in Jülich was built in cooperation with the FH Aachen and commissioned at the end of 2008. Here is to investigate whether in Germany such a technology is useful. The 60 -meter-high receiver converts the 2000 mirrors reflected solar radiation at 700 degrees Celsius to steam generation. The 23.2 million euro expensive pilot plant produces an electrical output of 1.5 megawatts.

Case wind power plant

In contrast to the above-mentioned solar tower case wind power plants do not require solar panels on the ground, which concentrate the sun's energy to a certain point. With this principle, only a high fire is used, in the upper portion of water is sprayed. The evaporating water removes heat from the air, it cools to about 12 ° C above the outside air and falls within the chimney with velocities up to 20 m / sec downward. At the base of the chimney coupled to a turbine generator is installed as in the solar tower KW, which is driven by the wind generated artificially. The best and most consistent conditions for this type of power plant can be found in the horse latitudes, since it is open all year warm and dry air is available. Due to the indirect solar gain the technology works even at night. Towers with about 1200 m height and 400 m in diameter should reach at appropriate locations capacities of up to 900 MW or can provide a net power of 600 MW to approximately 2/3 of the year. Case wind power plants would thus on a performance and service life, which is comparable with conventional fossil and nuclear power plants. Although the efficiency is only about 2,5 %, but due to the "infinite " and free resource " warm air " financially immaterial, but wherein a larger area is required. About 1 /3 of gross electricity generated is required as the pumping energy to transport the water to be evaporated to the top of the chimney. Since the efficiency is greatly affected by smaller plants, this means that only large systems appear economically viable. Case wind power plants currently exist only as a concept. A realization is promoted in Israel, but fails at the moment of lack of financial resources.

Parabolic trough power plant

Here concave mirrors are used to concentrate the sun's rays to a point and thus to reinforce multiples. A mirror with a parabolic cross-section are particularly suitable for this, because they also are able to focus the radiation at the edge centers. If the mirror designed in the form of a trough, the solar radiation is concentrated about forty times, are directed onto an absorber pipe with heat-conducting liquid. To increase the performance they are arranged in north-south direction and can be tracked by an adjustable longitudinal axis in the daily course of the sun from east to west. The heat-conducting liquid is heated in their circulatory system to 400 ° C and produces over turbine and generator power. A well-known large-scale plant is the parabolic trough power plant in California's Mojave Desert. It has a total of 2.3 million square meters ( 2.3 km ²) mirror surface and generates 354 megawatts of electricity. Similar large-scale plants are planned at Crete, Egypt, and India. A further development of the so-called parabolic mirror fresnel collectors. For them, the sunlight over several arranged on the ground floor parallel ungewölbte mirror strips are bundled ( on the principle of Fresnel lens ) onto an absorber tube. The strips are a single axis. An additional secondary mirror behind the tube directs the radiation to the focal line. This concept is currently in the practical testing phase.

Paraboloidkraftwerk ( Dish )

In a Paraboloidkraftwerk a biaxially the sun tracked parabolic mirror focuses the sun's energy directly to an absorber is installed at the focal point of the mirror. The working gas (helium, air) is heated to up to 900 ° C and drives a Stirling engine or a turbine next to the absorber to. The Stirling engine converts the thermal energy directly into mechanical work. Such systems achieve the highest efficiency in the conversion of sunlight into electrical energy. In an experiment in France with a parabolic mirror of 8.5 m diameter (area 56.7 sqm), a net power of 9.2 kW was achieved, which corresponds to an efficiency of 16%. The modules are suitable for decentralized energy supply in remote regions and allow any interconnect many of these modules into a large-scale solar power plant. Thus, a power range can be covered up to a few megawatts.

Solar pond power plants

In solar pond power plants shallow salt lakes form a combination of solar collector and heat storage. The water at the bottom is much more saline and thus denser than at the surface. If solar radiation absorbed in the deeper layers, these heat up to 85 ° -90 ° C. Due to the existing by the different salinity density, the heated water can not rise, there is no convection takes and the heat is stored in the bottom water layer. The stored heat can be used to generate electricity in a turbine generator block and is an appropriate design 24 hours a day.

In addition to the end product of electric current there is the possibility to use the thermal energy in the solar chemistry. An important issue for the solar hydrogen economy research result is the recently at DLR successful thermal dissociation of water vapor into hydrogen and oxygen ( see also hydrogen production ). With the aid of a catalyst, the temperature required for this operation of several thousand degrees Celsius, could be reduced to less than 1400 ° C.

See also: Solar thermal power plant

Solar Cooling

Survey

Analogue for heating purposes can also be used for cooling solar thermal energy. Thus, the primary energy conversion into secondary energy is used (eg electrical energy), which simplifies the construction of the cooling system and saves costs. Since the highest cooling demand in typical applications often coincides with the time of highest solar radiation to solar heat is ideal even without caching as a propellant energy for cooling systems. Night storage and seasonal heat storage also allow a solar thermally driven cooling at times, in which no or there is not enough solar radiation available.

Solar cooling can be applied in principle, always and everywhere, where solar thermal energy can be utilized industrially. Prerequisite therefore is the structural suitability of the building for the operation of solar collectors, ie shading freedom, suitable roof orientation, and simply sufficient space on the roof. Whether a sole solar cooling already sufficient or needs to be supplemented by other methods, ultimately decides the only attachable collector compared to the cooling requirements.

Method

Generally there are a variety of technical processes for the conversion of heat into cold, so ultimately the driven by heat extraction of heat. The diagram illustrates these relationships. Many methods are already used practically ( in the graph marked in green), others are still in development ( marked in gray ).

Promising approach is focused on the use of long-standing tested absorption chillers and adsorption.

Application

Solar cooling is used, for example, in the following areas:

• Solar air-conditioning of buildings at high external temperatures
• Cooling of food, drink
• Industrial process cooling, for example in the chemical industry

In the climate of function rooms, machines, data centers and industrial facilities where high heat is obtained by the operation itself, in addition to the heat from solar thermal, these internally generated heat can often be used at other locations for cooling. Thus, the waste heat of the engine is used in conjunction with a thermoelectric element for the recovery of power, for example, in newer cars. In location-based applications, this same heat could be used to operate the air conditioner. It should therefore be examined in each individual case whether other heat sources are already available, the heat energy can be used for cooling.

Since solar thermal systems can be used in our latitudes during the winter for heating, it makes sense to use the same system for cooling in summer and heating in winter. To this purpose, a correspondingly larger dimensions of the collector system, and an existing seasonal heat storage can then be used in the summer for intermediate storage for night-time operation of the cooling. This results in a cost advantage over the operation of heating and cooling, each with separate cache.

Advantages of direct heat use

• It is the conversion stage on secondary energy (electric current ) is saved, which would be necessary for electrically -powered cooling system. This increases efficiency and simplifies the system design. The absence of secondary energy intermediates is accompanied by a reduction in the demands on infrastructure in this regard, such as power grids.
• In the area of ​​solar air conditioning in buildings, the primary energy is predominantly then available, when the cooling load is greatest, since the solar radiation directly caused the need for air conditioning. Even refrigerators and cooling systems in the context of industrial processes have to work at the time the highest outdoor temperature against the increased heat and then consume correspondingly more energy that can be provided by the high solar radiation at the same time directly without any intermediate storage.
• Solar cooling causes no electric and magnetic fields and thus in contrast to electric cooling systems, no electromagnetic pollution. Therefore, it is in this regard harmful to your health.