Design of Indian SWHS

Types of solar heating systems

There are different solar heating systems with non-concentrating collectors used in India providing heat below 100°C:

Solar air heating systems: Air is used as heat transfer fluid and is heated in the collectors by solar irradiation. The air is either used to dry agricultural goods like fruits, or used to provide space heating e.g. in the mountain area with cold temperature but high irradiation.

Solar Water Heating Systems (SWHS) are used for:

  • Domestic hot water heating (DHW): DHW is heated in residential homes, hotels, retirement homes, or hospitals by solar systems of different sizes and types. Since the DHW is used in the evening and in the morning, usually the solar collectors heat the water in the DHW storage tank, which is storing the DHW demand of one day.
  • Industrial processes: A lot of industries have a high demand on low temperature water of e.g. 60°C for washing, cleaning or production processes. Alternatively, the solar heat is used to pre-heat water, e.g. as feeding water for a steam boiler.

Typical designs for SWHS for industrial processes (SHIP)

In India, the design of SWHS for industrial processes (SHIP) often differs in comparison to other global markets, e.g. in Europe or USA. Among others, following types of SHIP systems are often used:

Thermosiphon SWHS

With increasing temperature, water expands, this means that it becomes lighter per unit volume. This effect is used in Thermosiphon SWHS, where the storage tank is installed above the level of the collectors. If the solar heated water becomes lighter than the colder water in storage tank it replaces the cold water in the storage tank. The temperature difference drives the natural circulation, which is perfectly controlled by nature and works very reliable. However, it is mandatory that the diameters in the hydraulic pipes are large enough and a free flow of the water without bottlenecks is given.

Forced circulated SWHS

If the water storage tank is installed below the collector field or the distance between tank and collector field is too big or the installation of the pipes doesn’t allow a proper natural circulation, pumps are used to circulate the water through the collector circuit. The pumps are usually operated by an automatic controller, but can also be switched on and off manually if stable and similar weather conditions exist.

Non-pressurised, open systems with water as heat transfer fluid

Indian SWHS are often non-pressurised, this means, there is no additional pressure on the hydraulic system than the static pressure. Water is usually used as heat transfer fluid, since in most areas of the country freeze protection is not needed. If treated water through, for instance reverse osmosis (RO), is used which is available in the industry at least as feeding water for steam boiler, then the SWHS gets protected against corrosion. Since the water volume is expanding with increasing temperature, an expansion vessel is needed at the highest point of the system to incorporate the increased water volume.

Non-pressurised SWHS with expansion vessel

Non-pressurized systems have a lot of advantages: they are less costly, since the pipes, the fittings and the components of the hydraulic circuits must not withstand higher pressure, a membrane expansion tank can be avoided and there is no need to control and assure a specific tightness and pressure in the hydraulic circuits. If a glycol-water mix would be used as heat transfer fluid, leakages and losses of the fluid via evaporation by the open expansion tank would cause a problem, but with water as fluid, it doesn’t matter. However, since air comes into the hydraulic circuit in open SWHS, it is important that the air can escape via air vents to avoid bottlenecks for fluid flow in the network.

Open-loop and closed-loop collector circuits

In a lot of countries, the heat transfer fluid is circulating in a "closed loop" through the collector circuit, this means, it is decoupled from the process water circuit by a heat exchanger. This is necessary, if freeze-protection by a non-freezing heat transfer-fluid is needed, if the process water hydraulic system stands under high pressure or if the purity requirements for process water don’t allow, that it flows through the collectors.

In India often "open loop" SWHS are used, where the process water flows through the collectors and is afterwards used in the process. This is possible, since it is not unusual in India, that residential and company buildings are either not connected to a public water piping system or, if connected, the supply is erratic. Many times, water is also supplied by tank lorries or other sources. Therefore, there are water tanks to store supply water on the roof without additional pressure and since in most parts of the country anti-freezing is not necessary, the collector circuit must not be decoupled from the collector circuit by a heat exchanger.

Open loop SWHS are cheaper by avoiding heat exchangers. However, there is the challenge of corrosion of the pipes, the collectors and the tank, if aggressive water is used. Companies often run their own reverse osmosis (RO) water treatment system, which also protects against corrosion of the solar system.

Open-loop SWHS (process water is flowing through the collector)
Closed-loop SWHS (heat transfer fluid of the collector circuit is separated from the process water by a heat exchanger)

Batch SWHS

The design of SWHS in India is sometimes based on the concept to heat up a specific water volume per day to a specific temperature. This is why the SWHS are called “LPD systems” (litre per day systems). This concept leads to the SWHS design of a storage tank with a one day water demand volume. The tank is filled in the morning with cold water, which is heated during the day by circulating through the collectors and transferred in the evening to another storage for use.

This design concept is simple and is working, as the monitoring results of the Synthokem SWHS proofs. However, the concept is rather inflexible with regards to changing hot water demand and varying solar irradiation – daily and seasonal. It causes additional heat losses by longer storage time of the hot water, since it is used only the next day instead of being used directly. In addition, it is difficult to automate the concept.

Example of a batch system at Synthokem Labs: the water of the “Night demand” storage tank (left) is transferred to the “Day demand” storage tank in the evening