3.1 Solar water heating systems [ctd.]

Worldwide Potential of solar water heating

It is often argued that solar energy is not a potential alternative to rapidly depleting fossil fuels.

And it is a commonly accepted misconception about solar energy that…

Solar energy has nothing to do with rapidly increasing energy demand!

However, there are plenty of examples that strongly urge us to through above misconception away.

For instance, the cumulative solar thermal energy conversion capacity reached a milestone of 375 TWh in 2016 [1].

Think!

375 TWh of solar thermal energy means

• We have been able to save as much as 32 000 000 tons of oil just within 2016 thanks to solar thermal technology. It is enough to shut down as many as 70 coal-powered power plants.
• We have been able to cut as much as 131 000 000 tons of carbon dioxide just within 2016 thanks to solar thermal technology. As many as 7 million tree seedlings grown for ten years would be needed to sequester such amount of carbon dioxide.

……One clear example to demonstrate the practical power of sunlight!

That being said, the most popular application of solar thermal energy is solar water heating. Therefore, the credit should largely go to solar water heating.

Technology & configuration of solar water heating systems

Solar water heating systems rely on solar energy to produce hot water for industrial and domestic uses. Several technologies have been developed and available on the market with different configurations. We can choose them depending on our requirements, budget, and local weather conditions. A typical solar water heating system is consist of a collector, a heat exchanger, a control system and a heat storage tank.

Collector

The collector is the heart of a solar heating system which is excellent at collecting solar energy effectively even on overcast days and converting absorbed energy into useful heat. The heat is then transferred to a heat-transfer liquid or gas. Several types of collectors have been developed namely flat plate collector, evacuated tube collector, evacuated flat plate collector, selective absorber and flat-plate evacuated. While flat plate collectors are the most popular, evacuated tube collectors are also being widely used.

Flat plate collector

A flat plate collector (figure 01) is consist of a front cover (transparent), heat absorber and a housing. The absorber is the key component of a system which converts sunlight into heat and transferred to water inside the absorber tubes. The absorber material should be essentially good at absorbing heat and must be highly heat-resistive. Therefore, heat absorbers are usually made of copper, steel or aluminium.

Figure 01: Flat plate collector

Evacuated tube collector

Flat plate collectors (Figure 02) are more economical than any other type of collectors but heat loss due to convection is overwhelmingly high. Evacuated tube collector concept avoids the convective heat loss by maintaining a high vacuum around the absorber.

Figure 02: An evacuated tube collector

An evacuated tube consists of a heat piped inside which is a heat transfer liquid or gas (Figure 03). The pipe has been enclosed with a glass tube. The high vacuum between the glass tube and the heat pipe avoids convection heat loss and thus enhances the overall efficiency of the system.

Figure 03: An evacuated tube

The sunlight heats the fluid or gas inside the heats pipe and evaporates. The vapor rises up and enters the heat exchanger at the end of the evacuated tube where heat is delivered to water with an antifreeze agent. The vapor condensed back into its original form (liquid or gas) once its heat is transferred and returns to the heat pipe where it is heated again by sunlight maintaining a continuous heating cycle.

Evacuated tube collectors can achieve very high temperatures but require small collector areas than flat plate collectors. Therefore, evacuated cube collector suit better than flat plate collector concept, especially for cold weather conditions.

Heat exchanger

The role of a heat exchanger is to transfer heat absorbed by the collector to water. Therefore, the material used to design heat exchangers must be good at conducting heat. Copper, aluminium and bronze are some of the good candidates. Anyway, copper has become the most common heat exchanger material owing to its higher heat conductivity and corrosion-resistivity.

Hot water storage tank

These are available in different sizes. They have a well-insulated body to store hot water and retain its heat for days so that hot water is available whenever it is needed.

Pump control system

A solar water heating system maintains a heating cycle in which water in the storage tank is heated systematically. This circulation of the heat transfer fluid is maintained by an electrical pump. Water would be continuously heated whenever fluid in the collector is hotter than the water in the storage tank. However, instead of getting heated water would be cooled in the case where the collector is colder than the water in the tank (early in the morning and evening). The pump should be turned off to stop water getting cooled when the collector is colder than the water in the tank. And it should be turned on when the collector is hotter than the water in the tank.

A pump control system is a smart device which monitors the temperature of both collector and water in the storage tank and turns on or turns off the pump accordingly avoiding water getting cooled and heating water in the tank whenever it is possible. Usually, the pump is turned on when the temperature of collector is at least 5 ⁰C higher than that of water in the tank.

Anyway, there are some configurations (thermosyphon solar water heating systems) that do not need a pump control system in their operation.

Different techniques used to heat water

Currently, solar water heating systems use two main techniques to heat water.

• Forced circulation

Most of the solar water heating systems need an external power source to keep circulating the heat transfer fluid through the system. Usually, it is satisfied with inbuilt solar panels or power supplied by the grid.

• Thermosyphon

It is a rule of nature that cold water goes down whilst warm water rises due to the fact that density of cold water is lower than that of warm water. Thermosyphon systems are designed to make use of this natural phenomenon to circulate heated water through the system. As such, these systems require neither an electrical pump (to circulate a heat transfer fluid) nor an external energy source (to run a pump).

The tank is fixed above the collector so that cold water sinks down to the collector and absorbs heat at the collector. The density of water drops upon absorption of heat at the collector and thus rises to the storage tank. Meanwhile, cold water from the storage tank sink down to the collector since the density of cold water is lower than the warm water. The rise of hot water to the storage tank and dip of cold water from the tank to the collector keep running the circulation of water through the system and keep heating water continuously as long as the system receives sunlight.

Different configurations of solar hot water systems

Some systems require more collector area but cost you less (flat plate collector concept). Other designs may cost you much but may much efficient at collecting heat. Therefore, such a system suits better if you live in a region where the sky is often cloud covered. The system configuration should fit the local weather conditions, inclination of the rooftop, available rooftop area and your pocket as well.

Advantages of solar water heating systems over conventional electric water heaters

As a sustainable approach, a solar water heating system brings you a range of benefits.

• The key advantage of having a solar water heating system on the rooftop is the amount of electricity (and money) it can save when we switch to solar energy for producing hot water.
• They utilize renewable energy from sunlight which is free and abundant.
• Shrink load on the grid and help reduce the amount of fossil fuel burning/ emissions
• Unlike other renewable technologies, solar water heating systems employ simple configurations/ technologies. As such, solar hot water systems have been manufacturing not only in developed countries but also in developing countries such as Pakistan, India, Bangladesh and Sri Lanka.

Being an economically successful solar energy technology, solar water heating concept inspired the researchers to develop more successful solar energy technologies. It could discard a variety of misconceptions and overcome a range of economic and technical barriers. Successfully!

It is true that solar water heating technology is still economically and technically viable. However, it seems to be uncompetitive with the falling price of solar photovoltaics.

The falling price of solar photovoltaics forces us to think of solar water heating twice.

Solar water heating or solar photovoltaics?

Let us discuss this issue in a separate article, in details.

Reference

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