Solar heating system with "TERMO" combi system (yellow, 120 mm insulation; stabilised stratification by thermal flow funnel). Wagner & Co, Cölbe

Solar Heating Systems often contain two storage tanks: the Buffer Storage Tank to provide storage capacity for Backup Heating (space heating) purposes and the Hot Water Tank to heat potable water used for bathes, showers and so on. To save space and to avoid the need for complex regulation strategies controlling the distribution of the thermal energy between both storage tanks, so called Combi Systems have been developed, which solve both problems by their design.

Among the most widely-used types of combi systems are tank-in-tank storage systems. They consist of a big buffer tank containing the heating-circuit water; inside of this big tank a second, smaller inner tank containing potable water is used to provide hot water. So the solar system heats heating-circuit and potable water simultaneously, the distribution of warmth between both circuits is governed automatically by the design of the tank. An Heat Exchanger tranfers heat from the Collector to the heating-circuit water in the outer shell; the surface of the inner tank transfers heat from the heating-circuit water to the potable water.

The most simple design is to hook a small potable water tank into the top section of the buffer tank, where the water is hottest (see Thermal Stratification). The main problem with this design is to avoid turbulences within the thermal strata caused by the inflow of cold potable water, which could damage greatly the efficiency of the solar system. To minimise this problem the inlet pipe will often enter the buffer tank at the bottom and lead through the whole buffer tank to the top, so preheating the potable water en-route.

A logical enhancement of this concept leads to inner tank which is formed like a mushroom with a wide head and a somewhat smaller shaft placed in the center of the outer tank. Passing the "shaft" the potable water is heated without disturbing the stratificiation in the buffer tank much; the head provides a sufficient reservoir of hot water. Other concepts use conical or plain cylindrical inner tanks, reaching from the bottom to the top of the outer tank; in the most applications the differences in efficiency between this concepts are not significant. As a rule of thumb it is recommended to dimension the inner tank to about a fourth or third of the total volumen of the tank-in-tank system.

Solar collectors, modules and components are being tested by universities, magazines and further institutions. The results are accessible on different ways, partly for free, to some extend liable for costs.

Voluminous tests of solar heating systems and collector presents the Swiss “SPF Institute” at Rapperswil on its website. It offers comprehensive services related to thermal solar collectors. These services are useful to manufacturers as proof of the performance of their products, for quality control and as marketing instruments. The measurements and results help the system designer and the building services engineer as a basis for correct system dimensioning. The published test reports provide orientation within the market to the end user: http://www.solarenergy.ch/e/spf/index.htm

Further Information is provided by the Florida Solar Energy Center, a research institute of the University of Central Florida.
Testing and certification of both solar water and pool heating collectors is a State of Florida mandated activity. All collectors and systems sold or manufactured in Florida must be certified by the Florida Solar Energy Center (FSEC). The FSEC testing program evaluates solar collectors to determine that they meet the certification standards developed by FSEC.
Collector-Tests under http://www.fsec.ucf.edu/solar/TESTCERT/COLLECTR/Collectr.htm.
Solar heating systems tests: http://www.fsec.ucf.edu/solar/TESTCERT/SYSTEMS/Systems.htm.
The Florida Solar Energy Center also offers a list of approved photovoltaic systems and information on FSEC-approved grid-connected PV systems. Included are sizes, model numbers, major components, supplier contacts and documentation associated with maintaining approvals: http://www.fsec.ucf.edu/pvt/BuyInstallPV/pvapprovals/approvals1.htm.

Solar heating system. LgaBW

Thermal solar use means the transformation of Solar Radiation directly into useable heat. Specific areas include Passive Solar Design in Solar Architecture, the Standard Solar Technology used to heat potable water and for space heating, and the Parabolic Mirror (representing Solar Thermal Electric Power Plants) for the industrial production of processing energy and electricity.

Solar Heating Systems based on the Standard Solar Technology are the most wide-spread technology of active solar heating (in opposition to the "passive" utilization of Solar Radiation by means of Passive Solar Design in Solar Architecture).

A Collector transforms solar radiation to thermal energy by heating a liquid (the so-called Solar Fluid) pumped through the collector. Controlled by a Solar Regulator the heat is transported within a pipe system (the solar circuit) to the Solar Storage Tank and transferred by an Heat Exchanger to the storage medium (water, in the most cases). As a security device an Expansion Vessel maintains the pressure balance in the solar circuit.

Solar tank with optimised stratification, 2000 litres volume. Sonnergie.

The principle of thermal stratification in solar storage units is – besides the insulation of the storage tank – of very high importance to the efficiency of a solar heating system.

The thermal stratification is based on a natural process: Since warm water is lighter than cold water, it will ascend until it reaches a layer of warmer water or the top of the tank. This process facilitates the efficient utilization of solar heat: The higher the temperature difference between Collector and Solar Storage and the longer such a difference exists, the higher the Efficiency of solar heating. Therefore the solar Heat Exchanger will be mounted near the bottom of the tank, where the water is relatively cold, so even small amounts of solar heat can be "harvested". And while the outlet will be near the top of the tank, where the temperatures are highest, the inlet feeding fresh cold water will be positioned near the bottom. The more stabile the thermal stratification, the higher the efficiency of the solar heating system and the comfort for the user by providing reliably sufficient amounts of hot water.

While all state-of-the-art solar storage tanks make use of thermal stratification, there are signficant discrepancies in design and efficiency.

The most basic variant is represented by an so-called bivalent, upright-standing hot water tank for solar systems. Such an tank should be tall und have a relatively small diameter to allow significant stratification. The term "bivalent" relies to the number of heat exchangers integrated in the tank: The "solar" heat exchanger transfers the solar heat to the cold water at the bottom of the tank. An second heat exchanger, mounted at the top of the tank, serves as backup heater (electrical or connected to a central heating system), if e.g. in winter the energy provided by the sun not suffices to produce as much hot water as needed. A thick insulation minimises the cooling-down near the walls of the tank and thereby thermal turbulences in the tank caused by such local heat losses.

More sophisticated design concepts include different additional measures to further the thermal stratification and to avoid thermal turbulences. Some systems add a third heat exchanger in the middle part of the tank ("trivalent tanks"); an electronical regulator then uses pre-programmed strategies to distribute the solar heat between the different heat exchangers. Other concepts try to achieve similar effects solitary by the design of the interior of the tank; a wide-spread implementation of this idea are the so-called Thermosiphonic Tanks (not to confuse with Thermosiphon Systems). Many types of tank design optimising thermal stratification are specially conceived to integrate in Low Flow Systems.

Tanks with optimised thermal stratification boost not only the efficiency of the solar heating system, but also the comfort provided to the users: Starting with a totally cooled-down (unloaded) storage tank they will be able to supply solar heated water at significantly shorter notice than non-optimised systems.

Schematic diagram of a thermosiphon system. Illustration: Heindl Internet AG

Usually, the Collector is placed on the rooftop and the water tank in the basement. This means that circulating pumps have to transport the heat-transfer fluid from the tank up to the collector in order to keep the entire system working.
Thermosiphon systems work differently: the water tank is also mounted on or under the roof, but, most importantly, it is above the collector. This set-up makes it possible to use gravity for circulation.
The principle is simple: the Solar Radiation heats the heat-transfer fluid, whose density then decreases as its temperature increases. The fluid becomes lighter and rises – a phenomenon known as natural convection – inside the circulation pipes. An extra pump is not necessary. A Regulator, or Controller, is also not necessary because the sun controls the flow of the heat-transfer medium: When it shines, the heated heat-transfer fluid rises in the standpipes. The potable water is heated over a Heat Exchanger with a large surface area. The heat-transfer fluid which has cooled down then flows back to the collector – the process is back where it began.

Thermosiphonic tanks are Solar Storage Tanks which optimize their internal Thermal Stratification by using and encouraging thermal gravity effects. They are not to confuse with Thermosiphon Systems which are replacing pumps in the solar circuit by gravity effects to power the heat transport between Collector and storage tank.

At the center of a thermosiphonic tank, from nearly the bottom to the top, runs a wide thermal conduction pipe, at the base set above or around the solar Heat Exchanger sitting at the bottom of the tank. This pipe works as a convection funnel, in which the water heated by the heat exchanger ascends to the top section of the tank without disturbing thermal stratas in the already heated water surrounding the pipe system. Special designed outlets in the wall of the pipe enable the newly heated water to discharge in the surrounding tank as soon as it has reached a layer (or strata) having nearly the same temperature as itself.

One advantage of these systems is the low thermal inertness: nearly as soon as the sun is heating the collector warm water will ascend to and accumulate in the so-called stand-by section (meaning the top portion of the tank), ready to be used. The second advantage is the optimized stratification with minimized turbulences without the need of an elaborated electronic regulation system, allowing an highly efficient operation of the solar heating system.

Three-Liter-Building

As an especially economical variant of low-energy buildings (the new standard for new buildings), the descriptive concept of the “Three-Liter-Building” is analogous to the “Three-Liter-Car”.

Heating needs of buildings in kilowatt-hours per square meter per year:

Three-Liter-Building 16 -- 39 KWh/m²*a

Low-Energy Building 40 – 79 KWh/m²*a

Passive Energy Building max. 15 KWh/m²*a

Zero-Energy Building/Energy-Producing Building 0 KWh/m²*a or energy surplus

Existing Buildings Depending on Insulation 80 – 300 KWh/m²*a

In this case, a quantity of heat corresponding to 10 KWh =: 1 liter of heating oil, 1 m³ of natural gas or 2 kg of wood pellets. Therefore, in a three-liter-building only about 3 liters of heating oil would be used per square meter of living area each year.

(It is true that the expression Three-Liter-Building in itself says little about any type of ecological building method)

The time span that a Solar Power System requires to produce as much energy as was needed for production of the system. A typical Solar Heating System available on the market will payback (or amortise) after about four years, while such a system is expected to function from 25 to 30 years.

The energy payback period of photovoltaic systems is comparable. PV Modules based on amorphous silicon have an energy payback period between 17 and 41 months according to a study done by TU Berlin (German). Traditional power plants are unable to amortise due to the fact that they continually require fossil fuels.

Biaxial tracking photovoltaic system on a stand. Type: Pylon PN21. Picture: Sun Technics Solartechnik GmbH, Hamburg, Germany

By using a Tracking System, a photovoltaic system’s annual production can be increased up to 30 % when the PV Module surface tracks the sun throughout the day.

Biaxial tracking photovoltaic system on a stand Type: Pylon PN21. Picture: Sun Technics Solartechnik GmbH, Hamburg

In photovoltaic tracking systems (heliostats), the surface of the module tracks the sun throughout the day. The Tracking can be along either one or two axes whereby tracking along two axes provides a higher power output. Compared to south-facing stationary systems, such a tracking system can increase the year’s total output by up to 30 % at 45° latitude. By setting tracking systems atop a mast one has the option of choosing the optimal site. No longer constricted to the disadvantages that come with surrounding Buildings, these systems have a 180° slewing range free of obstruction (i.e. shadows).

Transparent heat insulation, community in Erkrath, Germany. Picture: MBW.NRW

By using transparent insulating material combined with a large, black-painted wall with good heat conduction, a crude Absorber, an efficient, passive use of Solar Radiation is reached. The light passes through the transparent insulation and is then absorbed by the black-painted surface. The heat then flows directly through the walls to warm the rooms inside. An example of Passive Solar Design.

Collector field (90 m² of evacuated-tube collectors) to power the refrigerating machine of the University Hospital Freiburg’s (Universitätsklinik Freiburg) air-conditioning system. Picture: Fraunhofer ISE

Also called an Evacuated-Tube Collector. In this type of Collector, the Absorber is inside an air-evacuated glass tube, and, compared to a Flat-Plate Collector, energy loss can be further reduced and temperatures up to 150 °C (302 °F) can be reached. Because of their high Efficiency, evacuated-tube collectors also work on slightly cloudy days.