Solutions for the contribution to heating and obtaining domestic hot water
There are, in principle, two reasons why the number of combined solar installations which, apart from heating the domestic water also helps to heat the rooms: firstly, more and more houses with minimal energy consumption or passive houses are built, that is to say buildings of whose energy needs are low; Here you can install heating systems that operate at low temperatures, such as floor or wall heating, suitable for operation based on the solar installation. For heating systems whose supply and discharge temperatures are between 90/70 ° C, it is not possible to connect to an efficient solar system to help warm the rooms. It must be fitted to “low temperature operating heating systems” of supply and discharge between 60/40 ° C or less (can be adjusted throughout). On the other hand, innovations in solar technology have contributed to this. development, first of all through improved tank and system technology, so the solar system and the heating system are increasingly becoming a single unit.
In combi installations, the second heat exchanger makes it possible for the variety of plant configurations to be even greater compared to that of domestic water installations. Although only forced recirculation systems and three closed circuits (solar circuit, consumer circuit and heating circuit) are considered, there are a variety of variants that result in particular from the different hydraulic connection to the heating system.
In addition, heating systems can have several heating circuits, which are required when, for example, one part of the house is heated by heaters and another by floor heating. In this case, the two circuits also have different levels of temperature. The situation can therefore be quite complicated, especially in the houses already built, where over time changes have been made not only to the building, but also to the heating system. In complex cases like this one must be called an experienced specialist. In the following, it will always start from a heating circuit so that the variants of system configuration can be included in the analysis.
When choosing the system connection mode, the distinction must be made between the so-called modular heating boilers and the non-adjustable heating boilers, which have a relatively constant efficiency. If the boiler is a modular one, then the heating power can be adjusted to the heat requirement, while the non-adjustable boilers always require the installation of a tank that gives heat to the heating circuit. In this case, the heating circuit and the boiler circuit are hydraulically separated from each other; if the boiler efficiency is too low, then the tank with maximum efficiency can intervene; while if the heat requirement is reduced, the tank takes over the boiler efficiency and thus avoids too much synchronization of the burner.
The simplest solar system to contribute to the heating of the home implies that the solar collectors heat the coil of a buffer vessel that is connected to the heating system of the house and, optionally, to the conventional heating source. The logic of the intake system permanently monitors the temperature in the return of the heating system and the temperature in the buffer vessel. If the temperature of the buffer tank is higher than that of the return of the heating system, the solar controller will operate a 3-way valve that enters the heating circuit into the buffer tank. In this way, the heating system of the house will use the thermal agent prepared by the solar installation, ensuring only the necessary surplus up to the desired temperature. If it is desired that the buffer tank is also heated by the conventional power source, it is also possible to install a pump that can provide this function.
This diagram is similar to system 1, except that the storage tank is equipped with 2 coils that are connected in the solar installation, in order to stratify the heating agent. This technique is useful if the buffer tank is tank type in the tank, thus allowing priority heating of domestic hot water.
This diagram is similar to system 1, except that east-west collectors can be used. Two pumps will be used to control the collectors.
This diagram is similar to system 1, except that east-west collectors can be used. For the control of the collectors a pump and a diversion valve will be used.
This system combines the advantages of system 2 (vessel with two coils) and the possibility of mounting collectors oriented east-west.
For situations when it is desired to heat two tanks, the use of system 6 allows the solar fluid to be diverted between tank 1 and tank 2 using a diversion valve. The deviation can be controlled depending on the temperatures of the 2 tanks using successive heating techniques or parallel heating.
This scheme is similar to diagram 6, except that it uses a control system consisting of two pumping groups, without the use of a deflection valve.
System 8 is used in situations where two tanks will be heated and the panels will be located with an east-west orientation. Two pumping groups and one bypass valve will provide the logic of heating the solar system.
Very large quantities of fossil fuels are needed to heat the house. Therefore, the solar heating of the rooms, total or partial, plays an increasingly important role for the solar collectors, because in this way the largest quantities of fossil fuels can be saved. It is also a disadvantageous solution, because the demand and the supply of solar energy do not always coincide.
In recent years, specialists and technicians in the field of research and companies have devoted themselves to the study of this technical challenge. Thus, in addition to increasing the surface area of the collector and the tank, the technology of the tank and the system in general has also been improved. Today, solar-powered systems are increasingly common. As regards the Europeanization process, all EU Member States are obliged to amend the directives on “the general degree of energy efficiency for buildings”. In support of this directive, EnEV2002 was issued the electricity saving decree (germ .: Energieeinsparverordnung), an integral part of the construction right in Germany (nt).
Within the member countries, the EU has introduced the so-called “passport or identity card of buildings”, where buildings – like refrigerators or other types of household appliances – are divided into energy classes. In this way, the use of solar systems is encouraged, since it is assumed that all buildings with solar installations will be better evaluated.
An important premise for a high contribution of the solar system is the low temperatures in the heating system (initial temperatures of 30-50 ° C), so significantly lower temperatures than in the case of older buildings (which sometimes reach up to 90 ° C). In old houses it is therefore important to improve the level of thermal insulation (to reduce the need for heating), and then to optimize the solar heating system, including the heat distribution system (hydraulic and surface heaters). The aim is to reduce the water temperature in the heating system as much as possible.
On the other hand, the problem of solar heating in old houses cannot be overlooked: the heating period is limited to the winter months, from November to February / March, that is, in the months with the least amount of solar radiation. This causes a higher demand for the installation.
Although it has larger collection areas and a proper operating technique, a solar installation for complementary heating consists, in principle, of the same components as a system for heating domestic water. The surface differs depending on the degree of coverage; thus, for households with a family (with low energy consumption), for a 100% coverage of the energy requirement it may be necessary that the surface of the collector has a view of 50m2 and a storage volume of 30m3 or more. However, the full coverage of the heat requirement, including the heating of the rooms by solar energy, is an exception. A collector with an area of between 8 and 20 m2, with a tank of 600 up to 2,000 l, can cover between 20-30% of the energy needs of a house for a family.
The simplest concept is to increase the volume of the tanks, which can be achieved by installing a buffer tank. However, such systems require more effort in regulating and connecting the tubes, as well as relatively much space. Therefore, heat storage systems have been imposed on the market, for example, in the form of so-called combi-type tanks, which contain a small domestic water tank on the top, securely fastened to the top of the buffer tank. There are also reservoirs with layers, where both charging (solar energy) and unloading (for domestic water) take place through a heat exchanger. This type of tank is characterized by a well-developed system (which works for example on the “low-flow” principle) and which allows optimum use of solar energy. In recent years, the number of newly installed solar installations has increased significantly.
In addition to small, individual installations, large installations, such as those that provide the energy needed for multi-family homes, or those for entire settlements, can lead to lower costs of solar energy. The very large heat tanks have a volume between 1,000 and 100,000 m3 and, in relation to the volume, the installation is less expensive, and the heat losses are lower than for the tanks for households with a family. Such solar installations, along with correspondingly large collecting surfaces (large series products and even roofs whose entire surface is
they consist of collectors, also known as “solar roofs”) can power large residential complexes or, if there are local power systems, entire settlements could be supplied with solar heat.
Between 1993 and 2002, within the “Thermo-solar 2000” program, about 60 large installations (with areas between 80 and 700 m2) were installed to heat the water in communities such as nursing homes, fireplaces, hospitals and so on With the offer, manufacturers also had to submit a performance guarantee. Long-term installation verification shows that almost all installations have complied with the performance guarantee. In the case of newly-installed installations, further checks are still being made. The results are very good. documented and appropriate.
Within the same program there were also installed 7 solar systems of local heat supply having short and long-term tanks (seasonal tanks). An important role was also played by the fact that such installations were already in existence in Sweden and Denmark. In this case, all the households of a settlement are centrally supplied with hot water and heat based on some supply pipes (local heat supply system). The collecting surface of such installations differs between 100 and 1000 m3, the storage volume between 1000 and 20 000 m3 depending on the size of the collector and the concept of the tank (hot water, waste / water tanks, earth heat probes, reservoirs for water from groundwater). For example, in the case of one of the largest local heat supply systems in the Hamburg-Bramfeld region, a collection area of 3000 m2 was fitted, with a storage volume of 4500 m3 (hot water tank); The solar system thus covers 50% of the annual heat requirement for 124 houses with one family, with a total heating area of 14,800 m2.
In the meantime, the first solar heaters were also manufactured under a conventional remote power system. One such example is an installation in Marstal, Denmark, which has a collection area of 18.365m2 and is currently the largest solar installation in the world. It is equipped with a relatively small tank with a volume of 2,100 m3, so that the solar energy covers 30% of the energy requirement and directly supplies the heating system. In a similar way, the plant in Kungalv, Sweden has been operating since 2000, with a collection area of 10,000 m2, a storage capacity of 1,000 m3 and a thermal power plant based on biomass. The advantages of such installations are the small tanks and the low costs. On the other hand, the disadvantages are that, in general, heating systems operate with very high temperatures up to 100 ° C and above (steam systems), so they do not offer cost-effective operating temperatures for solar installations.
So far, there have been several pilot projects from public funds or sponsorships. Although from an experimental point of view they have been and are partially successful projects, there is still no tendency to extend the use of large systems.
Sizing of installations for domestic hot water and heating
Although the energy supply of the sun and the thermal demand do not correspond temporarily, the solar heating support installations, also known as combi installations, occupy a market sector of more than 30%. An essential role was played by the introduction of the EnEV electricity saving decree in 2001. Since then, decisively, it is no longer necessary to heat a house, but the entire primary energy requirement. This means that, when dividing buildings by energy categories, important are not only the variable architectural-physical characteristics depending on the insulation, windows, windproofness etc., but also the technical installations, the used fuel, the consumption of electricity etc. Solar installations have the ability to positively influence this balance. This fact has not, finally, led to the emergence of technical developments, especially in the field of storage technology. Because heat storage is the key to solar room heating. A significant feature of combi installations is the multi-day or long-term tanks as opposed to the one-day hot water tanks.
For the installation of solar installations with supporting role of the heating system it is recommended that the following requirements are met:
- Low energy requirement of the building
The lower the heat requirement of a building, the higher the solar coverage required by a solar installation. At a high degree of thermal insulation of the building, the need for room heating is comparable to the energy requirement for water heating; in passive houses, the degree of coverage can be even higher than the heating requirement.
The most efficient is to support the heating systems in the newly built buildings according to EnEV, having an energy heating requirement between approx. 30 and 70 kWh / (m2 * a). In this case, a combi installation can have a significant contribution in the spring and autumn, when it is still needed heating, although the amount of radiation is high, but also on clear and cold winter days. Since the heating period of passive houses is only in the winter months with a reduced amount of radiation, the possibility of solar support of the conventional heating system is reduced, respectively, which requires significantly larger tanks, also known as long-term or seasonal tanks. .
- Integration of the solar installation in the utility system of the house
The proper integration in the utilities system of the house as well as the optimization of the hydraulic ratios is due to the fact that solar installations have become part of the heating system in recent years.
If the heating is partially solar with backup systems, then the heating surfaces must be mounted for a high temperature, the aim being to obtain the lowest exhaust temperatures. The optimum temperatures for supporting solar heating are 50/30 ° C or 60/30 ° C.
If the heating is done entirely solar, then the supply temperature must be lower, 30 ° C or even lower so that the long term tank can reach the highest temperatures. In this case it is in principle about heating through the radiation surface. The importance of the low heating temperatures in the circuit is demonstrated by the following calculation: it is assumed that the tank is charged at 90 ° C. If the lowest heating temperature is 50 ° C – a common value for conventional heating systems – the tank can deliver an energy of 46 kWh / m3, and at a temperature of 30 ° C it is 70 kWh / m3.
- User behavior
Apart from the construction mode and the utilities of the house, an important role for the heat requirement of the house and, therefore, of the solar coverage, is played by the user’s behavior; the smaller the heat requirement of a building, the greater the influence of the consumer. All the other indications regarding sizing started from an average user behavior.
- tilt of collectors
In order to support the heating system, a high degree of inclination of the collectors is recommended, for example, between 60 ° and even 90 °, in the latter case it is therefore about collectors of the facade.
- Type of collector
For solar support of the heating system, it is recommended to install vacuum tube collectors rather than flat collectors, because the former have better insulation properties in winter than in the warm period. However, the benefits of these facilities should not be overestimated.
In principle, solar installations with a supporting role of heating can have a degree of solar coverage of 0 – 100%. The following indicative examples show the size order of such installations is as follows:
- for the heating of a well insulated house are required annually approx. 4-6 I of fuel / m2 from the living area, corresponding to a value of 40 to 60 kW / a / (m2.a) almost exclusively from October to the end of March. For a house with a living area of 120 m2, this means a total consumption of 5,000 – 7,000 kWh / a of heating energy. The alternative would be to consider a passive house with a requirement of only 10 to 20 kWh / (m2.a) on an area of 120 m2, that is to say only 1,200 – 2,400 kWh / a.
- if the water tank covers half of the energy requirement of a low-consumption house (3,000 kWh / e) or the entire requirement of a passive house (1,500 kWh / a), the tank being charged during summer at 95 ° C and can be cooled to a minimum temperature of 25 ° C, then it would be necessary not to take into account the thermal losses of the tank – a storage volume of 37 m3 for houses with low energy consumption and of approx. 19m2 for passive houses (81 kWh / m2 storage capacity where T = 70 ° C).
If the thermal insulation of the tank is of very good quality (with a thickness of 30-50 cm) and a value (UA) of 1W / K, then the tank loses at the average temperature differences of 50-60 ° C approx, 500 kWh annually ; in order to compensate the losses from the tank, its volume should be increased by 6m2. For 50% solar coverage in homes with low energy consumption, therefore, tanks of approx. 43 m2, and for the complete heat supply of a tank house with a volume of approx. 25 m3 (NB: without considering the need for water heating).
the required area of the collector for this type of use should be large enough; high efficiency collectors with an area of 25-45 m2 should meet this requirement.
The example of the calculation clearly shows that heat storage and the need for heating are the essential factors when it comes to heating the rooms. But the same is not true for old buildings. The heating requirement for this type of building is so high in the months with a reduced amount of radiation that the expense for a collector installation including tank would be better used for the subsequent improvement of the thermal insulation.
In terms of dimensioning, it is good to differentiate between partial and total solar heating systems.
Partially solar heating
Partially solar heating refers to solar installations that provide less than 35% of the total heat requirement of the building. Installations of this type are suitable for houses built according to EnEV standards. According to the experiences so far, the following dimensions are recommended:
- Collector surface
– collector with an area of 0.8 up to 1.1 m2 / 10 m2 from the living area or
a collector with an area of 2 – 3 m2 / person;
– a vacuum tube collector with an area of 0.5 – 0.8 m2 / 10 m2 from the living area, or
a collector with an area of 1.5 – 2.5 m2 per person
- The volume of the tank
– an area of at least 50 l / m2 plus 50 I per person or
– an area of 50-85l / m2 from the surface of the collector or
– 100 – 200l / kW of heating capacity.
Some manufacturers of solar installations to support the heating system, meanwhile, also provide an interpretation chart specific to the manufactured product.
For a house whose surface is known (m2 of heated surface) the heating capacity can be calculated, and with the help of the diagram and depending on the size of the collector and the tank and the degree of solar coverage, the ratio between the volume of storage and the surface of the collector does not it must be less than 50I / m2, because otherwise it can reach a significant thermal surplus during the summer, which implies an overload of the solar installation and a specific lower efficiency of the collector. The dimensions of the aforementioned tank of 50-85 l / m2 from the surface of the collector must be calculated in such a way that the variations in the supply of solar energy can be compensated in a period of several days.
It is obvious that a larger surface of the collector and a larger volume of the tank also causes the increase of the solar coverage degree, which means, at the same time, the decrease of the degree of efficiency of the system and, at the same time, the decrease of the specific efficiency of the collector. . Neither should the problem be underestimated that the solar installation often stagnates during the summer due to oversize.
Total solar heating
Total solar heating means that the solar installation covers more than 70% of the heat requirement of the building.
A degree of solar coverage of this type is not utopian, and the concepts of the installations are no more complicated than those of the mentioned installations.
you so far. The essential difference is the size of the tank, as long-term or seasonal tanks are needed to store solar energy from summer to winter. The seasonal tanks are different from the usual ones, first by increasing them and secondly by the significantly better thermal insulation. It is recommended that the thermal insulation should be at least 50 cm thick.
The first house completely supplied with solar energy was built by the Swiss Jenni. For his house with a living area of 130 m2 and a heating capacity of 3 kW, he built a solar installation with a surface area of 84 m2 and a storage volume of 118 m2. Not long after, his system was shown to be oversized. According to the knowledge of those times it was already clear that a solar installation with a surface of 40 m2 (flat collector) and a storage volume of 30 m2 were sufficient.
The basic rule for sizing the installation
- surface of the collector: 1.5 to 3 m2 / 10 m2 heating surface
storage volume: 250 up to 1,000 l / m2 from the surface of the collector.
As shown by the bandwidth, very high solar coverage rates can be obtained with very different ratios between the surface of the collector and the volume of storage, where the necessary (heating energy + hot water consumption) is the decisive value.
Suppose that our family with 4 members lives in a passive house of the same size as the house with minimum energy consumption. The specific heating efficiency is 15 W / m2, respectively of approx. 2 W in total. Therefore, a specific energy requirement of 15 kWh / m2 results in a heating requirement of approx. 2,000 kWh per year. To this is added another 3,400 kWh / a for domestic water heating.
A total degree of solar coverage can be achieved with a solar installation whose surface of the collector is 35 m2 (2.7 m2 / 10 m2 of the living area) and a storage volume of 30 m2 (about 850 l storage volume per m2 from the surface of the collector). If the annual energy consumption is calculated according to the surface of the collector, then a specific annual energy gain of only 15G kWh / m2 from the surface of the collector.
Partial solar installations today correspond to a certain market sector and thus have reached a good technical maturity. Total solar installations have been tested by countless projects, while installations with a solar coverage rate between 35% and 70% have not been marketed at all, as the cost-benefit ratio is unprofitable. Despite the fairly high solar coverage rate, the conventional heating system cannot be lower or cheaper.
In addition to the simple heating of domestic water, it is particularly recommended for solar installations with a high contribution to cover the heat requirement, consulting an experienced planning specialist who can design and dimension the installation with the best cost-efficiency ratio. Last but not least, the available solar contribution is also a question of the costs that the user is willing to pay.