Solar Heating

Gunnar Boye Olesen, Copenhagen Environment and Energy Office

Though human cultures have used solar energy for millenniums, solar heating systems are a new technology, which has been utilized in Europe since the end of the seventies. Today so]ar heating plants are profitable in many situations in Europe. This is primarily true for plants heating domestic hot water and swimming pools, various solar drying plants, and simple passive solar heating design.

Energy from the Sun

To design solar heating systems, a general view of energy content, variation, and characteristic of the solar irradiation is needed. In Northern Europe the energy content is more than 10 times bigger during summer months than during winter months (in Southern Europe 5 times), and it varies appr. 20% between sunny and not very sunny years.

A great deal of the solar energy is In Northern Europe received as diffuse radiation, which means energy irradiation from the sky, and not directly from the sun. The diffuse radiation can be captured by flat plate solar collectors; but it can not be concentrated by mirrors. This is the reason, why concentrating solar collectors have a relative small output here, compared to other parts of the world, where the amount of direct solar irradiation is bigger.

Theoretically the optimum location of a solar collector is a south facing surface, with the same tilt angle as the latitude of the place (Denmark is located at 56› North). In practice there will always be shadows at the horizon, which means that the optimum is a slightly more level location. If the wish is to optimize on respectively summer and winter output, the tilt angle of the solar collector must be more level respectively more sheer.

A small deviation from the optimum orientation and inclination of a solar collector is not of practical importance, cf. the figure above and the following section on efficiency of solar collectors. In Denmark solar collectors are installed up to 60› from south.

A solar collector that traces the sun, will receive about 20% more solar radiation than a south facing optimum placed collector. This additional output does not compensate the costs related to a construction, which has to trace the sun. Usually it will be cheaper to install a 20% larger solar collector.

Active Solar Heating Installations

Domestic Hot Water Systems

The most widely distributed utilization of direct solar heating is for hot water production. An installation consists of one or more collectors in which fluid is heated by the sun, plus a hot-water tank where water is heated by the hot liquid.

In Northern Europe a solar heating plant can provide 50-70% of the hot water demand. It is not possible to obtain more, unless there is a seasonal storage. See the example at the end of this section. In Southern Europe a solar heater is able to cover 70-90% of the hot-water consumption. In Northern Europe a solar heating plant for hot-water has an energy pay back period of 3-4 years (see also chapter 4).

The simplest installations are the thermosyphon systems, with the storage tank placed above the solar collector. The temperature difference between the solar collector and the storage forces the circulation of the collector fluid, when the sun is shining, and heats the solar collector. This type of installation is popular in sub-tropical and tropical areas, especially units with integrated solar collector and storage tank. It is more difficult to utilize the thermosyphon solar collectors in Northern Europe, because of frost problems with the storage tank.

Solar water heaters are very popular in places dike Greece and Israel. They are now gaining a footing in Northern Europe, e.g. in Denmark where both the State and popular energy offices have put a lot of work into solar heating campaigns aimed at single-family houses.

For summer purposes there are many simple systems, that provide hot water when the sun is shining. Both self-builder systems and complete systems, e.g. for camping.

Solar Heating for Combined Space Heating and Hot-water

An active solar heating plant can provide hot water, and additional heating via the central heating system at the same time. To get a reasonable output, the central heating temperature must be as low as possible (preferably below 50›C), and there must be a storage for the space heating. A smart solution is to combine the solar heating installation with under-floor heating, where the floor functions as heat storage.

Solar heating installations for space heating usually give less profit than hot-water installations, both according to economy and energy, as heating is seldom needed in summer. But if heat is needed during summer, then space heating installations is a good idea.

In Northern Europe a solar heating plant can cover up to 30% of standard annual heat demand. In some places like the Alps, the total consumption can nearly be covered, as heat is demanded all year round, and winters are simultaneously sunny.

Solar Heated Swimming Pools

If one wants to heat up a swimming pool a few degrees above the outdoor temperature, a simple solar heating plant can be used, where the pool water is pumped through plastic collectors without cover. Due to low price and high output, this kind of solar collectors have become very popular in several places in Europe, first of all for outdoor swimming pools.

Solar Heating for District Heating Plants

Large solar heating plants for district heating are now in use, e.g. in Denmark and Sweden. Large so]ar modules for this purpose are constructed, which are practical to install directly on the ground in larger fields. Without a storage a solar heating installation can cover appr. 5% of the annual heat demand, as the plant must never produce more than the minimum heat consumption, including the loss in the district heating system (by 20% transmission loss). If there is a day-to-nighf storage, then the solar heating installation can cover 10-12% of the heat demand including transmission loss, and with a seasonal storage up to 100%.

Another possibility is to combine district heating with individual solar water heaters. Then the district heating system can be closed during summer, when the sun provides hot water, and there is no need for space heating.

Seasonal Storage

To cover the total heat consumption by solar energy in a house in Northern Europe, a storage that stores heat from summer and autumn is needed. Since water is a very efficient storage agent, a lot of experiments with large water storages have been carried out. Actually a house can be heated all year round by a solar collector combined with a super-insulated tank with the same volume as the house. But in practice this solution is far too expensive.

The most promising seasonal storages are large storages in connection with district heating, because large water storages are cheaper and with less loss per m than smaller storages. Some pilot and demonstration plants with seasonal storage have been built in Sweden. They have experience on seasonal storages in concrete tanks. in pools with insulated cap, in blasted rock caves, and in bore holes where the heat capacity of the soil contributes to the storage.

Drying of Crops and Houses

A solar collector that heats air, can be used as a cheap heat source for drying corn and other crops. The solar air-collector may consist of a black mat covered by a plastic plate. The air is drawn through the mat, where it is heated, and thereafter blown through the crops.

A damp house or room can also be dried out by blowing hot air from a solar air-collector into it. By using a photovoltaic driven blower, it can be secured that air is blown in only when the sun shines. Such installations are commonly used in summer cottages in Denmark and Sweden, where they keep the houses dry most of the year.

High Temperature Solar Collectors

If temperatures over 100›C are needed, e.g. for industry, or steam to generate electricity, there exist various possibilities with high temperature solar collectors. The most successful type is a concentrating solar collector made by Luz, a parabolic trough reflects the solar radiation to a black tube in the centre of the trough. This type is used at some solar power plants in California, but it would not be very efficient in Northern Europe, while it can not make use of the diffuse solar radiation. In Europe are manufactured flat-plate solar collectors with evacuated tubes, which can produce heat at temperatures from 100 to 200›C. Furthermore flat-plate solar collectors covered by air glass (an efficient, transparent insulation material), for the temperature range 100-200›C are under development.

Finally exist some pilot plants, e.g. in France and USA, consisting of a large number of mirrors that reflect the solar radiation onto a central absorber, where steam is produced and used for power production.

Design of Solar Heating Plants

Most solar heating plants are designed by simple hand rules or the f-chart method. As the exact consumption seldom is known, more exact computer simulations are not used for design, unless the plant is very big or very special.

Hand Rules

According to solar water heaters (heating from 8 to 45›C) with south facing, oblique solar collectors, which have selective absorbers, the following hand rules can be used:

* 1-1.5 m: solar collector area is needed per 50 ltr daily consumption of hot water

* the storage tank shall be 40-70 ltr per m: solar collector

* the heat exchanger in the storage tank shall be able to transfer 40-60 W/›C per mì solar collector at 50›C.

If these guidelines are followed, a solar water heater installed in Northern Europe will cover 60-70% of the annual hot water consumption, and will be able to produce 350-500 kWh/mì. With an installation like this, the additional heating can be turned off during 3-5 summer months, and idle loss from a furnace is cut.

Example:

A family with 4 persons uses 50 litre of hot water per person every day. It is heated by a furnace with an efficiency of 80%, and an idle loss of 500 kWh/month.

They choose a 5 mì solar collector (1.25 mì per 50 ltr daily consumption). The storage tank then has to be 200-350 litres.

They save appr.:

65% of energy for hot water	2000 kWh
idle loss, 4 month	2000 kWh
transformation loss, furnace	1000 kWh
(20% loss in furnace)
Total saving per year	5000 kWh
	= 500 ltr of oil

The family, which is living in Denmark, is going to change hot-water tank, and can choose between a new hot-water tank at 6,000 DKK, or a solar heating plant at 34,000 DKK, both prices include VAT and installation. As the storage tank in the solar heating plant can replace the ordinary hot-water tank, the family saves the money for this.

Net expenditure by choosing solar heating instead of just a new hot-water tank will be:

Solar heating plant	34.000 DKK
New hot-water tank, saved	+ 6.000 DKK
State subsidy, appr.	- 9.000 DKK
Net expense	19.000 DKK

The annual net saving will be:

Saved oil, 500 ltr at 4.20 DKK	2.100 DKK
Operation and maintenance, appr	+ 300 DKK
Total net saving, annually	1.900 DKK

For the family in this example, the solar heating plant has paid itself back in ten years (quicker if there is inflation). The expected life span of the plant is 20-30 years.

If solar collectors with non-selective absorbers are used, a 30% larger area is recommended. Low-flow installations and thermosyphon plants can produce up to 20% more than plants with a circulation pump. The guidelines above are though recommended also for these plants.

If a solar heating plant isn't optimum placed, then output and part of supply are reduced as shown in figure 8.9.

The F-chart Method

If a more detailed calculation than the guidelines above is wanted, the f-chart method can be used. With a small computer program based on this method, a solar heating plant can be designed quickly. In Denmark the most popular computer soft-ware of this type is the solar package from Energy and Environment Data. By using the f-chart method, it can be estimated how much a solar heating plant will produce month by month. For the calculations data are needed on monthly solar irradiation, outdoor temperature, the efficiency equation for the solar collector, heat loss from the storage tank, transmittance of heat exchanger, and consumption. The method is based on empirical figures for, how much a solar heating plant produces. Thus it can only be used for systems, where sufficient experience exists.

Construction of Active Solar Heating Systems

In the following the construction of a solar water heater is described. Solar heating plants for some other purposes are built up in a similar way.

The description gives some idea of the richness in details, one has to consider according to solar heating constructions. One should not be led to believe that it is possible to build a solar water heater just based on this description. One should only tackle this when experienced in plumbing, and in touch with people experienced in solar heating.

The Solar Collector

The main part of the installation is the solar collectors. They consist of a glass or a plastic cover, an absorber where the solar radiation is transferred to heat in the solar collector fluid, insulation along the edge and under the absorber, and a case that holds everything together, and allows the necessary ventilation.

When glass is used as cover, it is important that the iron content is low or zero, so at least 95% of the solar radiation passes through the glass. There are several disadvantages of using two layers of glass, so in practice always a single layer is used. If a plastic cover is used, it is important that the plastic can stand up to the UV-rays from the sun. It has been found that polycarbonate plates are very satisfactory. Often two layers of plast are used, e.g. channel plates can be used.

The absorber can be made of a plate with tubes where the collector fluid flows, of finned tubes (named sun-strips after the manufacturer), or simply tubes. The absorber is often covered with a selective black coating, which absorbs the sun rays, but holds back the heat radiation. Usually the absorber is made of copper, stainless steel, or plastic; sun-strips are copper tubes with aluminium fins. A lot of experience has shown, that absorber tubes made of ordinary steel or aluminium cause big problems with corrosion. It is essential that the absorber can stand up to the UV-light from the sun and the stagnation temperature (dry-boiling temperature), which is 100-140›C for solar collectors without selective coating, and 150-200›C with selective coating.

Usually solar collectors are mounted directly on top of the roof, or at a frame placed on a flat roof or the ground. Solar collectors can also be integrated in the roofing; but because of troubles with sealing between the solar collector and the rest of the roof, this is only recommendable, when a huge part of the roof is covered with solar collectors.

The Storage Tank

The storage tank shall store the solar heat. This is done by storing hot water until it is needed. The most efficient is a vertical tank with good temperature stratification, so the cold inlet water isn't mixed with the warmer water at the top of the tank. A horizontal tank reduces the output by 1020%.

The insulation of the tank must be so good, that hot water from a sunny day is still hot two days later. Especially the top must be well insulated, and without thermal bridges. It must be ensured that piping from the storage tank does not lead to self-circulation, which can drain the tank for hot water during periods without hot water consumption. If there is a flow tube pipe for the hot water, this must not be connected to the cold water, but has to enter at the upper part of the tank.

Usually the outlet of the storage tank is equipped with a scalding protection, so the water delivered for use never gets warmer than e.g. 60›C, regardless of the temperature in the tank.

The heat from the solar collectors is delivered to the water in a heat exchanger. As heat exchanger is mostly used a coil in the bottom of the tank, or a cap around the tank with collector fluid. In low-flow and self-circulating systems always a cap is used. In low-flow systems the solar collector fluid flows slowly down through the cap of the storage tank, which gives a stratification of collector fluid in the cap corresponding to the stratification in the tank. This gives more ideal heat transfer, and thereby a higher efficiency than. in traditional systems.

Solar Collector Circuit

The solar collector circuit connects the solar collector to the storage tank. The components of the circuit are:

* a pump that ensures circulation (not needed in self-circulating systems). The pump is usually controlled by a difference thermostat, so if starts running, when the solar collector is a bit warmer than the storage tank. If the storage tank has a heat exchanger coil at the bottom, a more simple control system can be used; e.g. a light censor, or a timer that starts the pump during day time.

* a one-way valve which prevents the solar collector fluid from running backwards at night, and emptying the storage tank for heat (not necessary in all kinds of installations).

* an expansion tank; either an open container at the top of the installation, or a pressurized expansion tank that contains minimum 5% of the solar collector fluid.

* overpressure protection (only in connection with pressurized expansion tank); must be a type that manages to let out the solar collector fluid, if the system is boiling. There must always be an accumulation tank for the fluid in case of boiling.

* air outlets, automatic or simply screws; must be used at ALL height points in the system, as air pockets will always appear.

* filling valve

* dirt filter for the pump, to remove dirt, e.g. from the installation (can be spared in some installations)

* manometers and thermometers according to the need

The solar collector piping must be of a non-corroding material. Systems with open expansion are most risky to get corrosion problems.

The solar collector fluid must be able to stand frost, and must not be toxic. Usually an approved liquid is used, consisting of water with 40% propylene glycol (can stand "20›C), and a substance that can be seen and tasted, if solar collector fluid leaks to the tap water. Also oil can be used as collector fluid, but it is very difficult to make a collector circuit with oil tight.

Operation and Maintenance

When the solar heating system is in use, there is not much to take care of. Once or twice a year it must be controlled that there is enough fluid and pressure on the system. Once a year it should be checked that the solar collector fluid hasn't become acid. Use indicator paper. Acid fluid should be changed.

In case the system is boiling, it is simply needed to fill new fluid on the system; as the old fluid may be damaged by boiling.

Some storage tanks must be decalcified? and the anti-corrosion zinc block shall be changed in appr. 10 years. it prolongs the life span significantly

Building Design with Passive Solar Heating The most simple form of passive solar heating is orientation of the windows, so all larger windows face south (+45›). A house with south facing windows needs 15-25% less heat supply than a similar house with east and west facing windows.

Saving is the largest, if the inner part of the house is built of heavy materials that absorb heat, and if the house has low energy windows.

Large south facing windows ought to be combined with shadowing overhangs that prevent overheating during summer. Passive solar heating design is popular in some places in the USA, e.g. in New Mexico, where there is barely built a house without passive solar heating considerations. More and more European architects use passive solar heating design for new buildings as well as renovation.

Glass Annexes

An unheated glass annex at a south front (south +45›); e.g. a greenhouse, a glazed-in balcony or patio, contributes to the heating. The heat saving is due to three conditions:

* the extra layer at the front insulates

* the sun heats up the glass annex, which further ;educe the heat loss from the facade behind the glass

* the air in the glass annex can be used as heated ventilation air in the house.

Roughly estimated, the glass annex can save half of the heat loss from the facade behind it. The saving totally depends on, how the house and the glass annex are used. If the doors and windows between the house and glass annex are not closed, or the glass house is heated, it may result in a higher heat consumption than without the glass annex.

The heat conservation due to a glass annex does not justify its construction, neither economic nor energy-economic (the energy pay back period is 10-14 years). The reason why glass annexes are still popular in Northern Europe, are the possibilities they provide, such as extra living space when the sun is shining, or a greenhouse; and they reduce the need for maintenance of the facade.

Solar Walls

A solar wall is a glass plate or transparent insulation on the outside of a wall. If the space between glass and wall is ventilated then the construction is called a ventilated solar wall. If the wall is heat conductive and a poor insulator, e.g. a solid brick or concrete wall, it will be heated during the day, and give off some of the heat to the rooms behind it in the evening and at night. A ventilated solar wall can contribute to the heating of ventilation air.

Solar walls have some distribution in the USA, e.g. in New Mexico. A number of demonstration installations with solar walls exist in Denmark and Sweden; but they are not very common in Northern Europe yet. The energy output from solar walls made of ordinary materials is 50-200 kWh/mì in Denmark, and a little higher when if of high-efficient insulation materials, e.g. air glass.

Solar walls are also known as trombe walls. named after the French Mr. Trombe who was one of the first to describe it.

Passive solar house with seasonal storage

If a house is built into the soil without insulation against it, the soil can be used as a seasonal storage. The ground must be insulated up to 6m from the house, and rain and ground water must be kept away. The temperature fluctuation in the house will not exceed 4-5›C between summer and winter, even with sun as the only heat source.

The system is developed in the USA, where it has been in use since the beginning of the eighties. The first house of this type in Denmark is built in 1991, and there is not much experience on, how this house type fits in North European climate.

Literature

1. Energihandbogen (The Energy Handbook), Judith Winther et.al. OVE's forlag, Copenhagen 1981.

2. European Solar Radiation Atlas vol 1. Commission of the European Communities 1984, EUR 9344, ISBN 3-88585-194-6.

3. Solar leafing i, Denmark small plants. The Danish Information Secretariat on Renewable Energy, 1991.

4. Byggehandbog for solvarme (Building Handbook on Solar Heating). Palle Ladekarl, Torben Skott. Aage Johnsen Nielsen. OVE's forlag, Copenhagen 1985.

5. Solar Heating in Denmark large plants. The Danish Information Secretariat on Renewable Energy, 1991.

6. Passiv solvarme projokteringsvejledning (Passive Solar Heating, design guidelines) Ministry of Energy, Solar Heating Program Report no. 30. Danish Technological Institute, Heating Technology and Institute of Thermal Insulation. Technical University of Denmark, 1985.

7. Passive Annual Heat Storage' Improving the design of Earth Shelters. John Hait. Rocky Mountain Research Centre, Missoula in Montana USA, 1983.

8. Solar Engineering of Thermal Processes. John A. Duffie, William A. Beckman. Wiley-Interscience publication, New York 19X0

9. Vedvarende Energy i Danmark (Renewable Energy in Denmark). Preben Buhl Pedersen, Jan Viegand and Niels 1. Meyer. Physical Laboratory Technical 111, University of Denmark, Lyngby 1990.