How Does A Solar Water Heater Work?

How does A Solar Water Heater Work - featured

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Every form of energy and heating under the sun comes from the sun – we sometimes forget that! Coal, oil and gas are all derived from organic growth nurtured by the sun’s rays. We simply convert the energy it contains into a form that we need for our civilization.

The renewable forms of energy such as solar PV electric, wave, wind and water heating also convert the sun’s energy but in different ways. Solar, wave and wind all involve secondary conversion, but it makes complete sense to gather the sun’s energy in one of its major forms – heat!

A solar heater transfers the sun’s heat to a liquid enclosed inside a tubular manifold and uses a heat exchanger to transfer that heat to an external appliance for use as domestic hot water. The circulating liquid is normally mixed with a non-toxic anti-freeze solution and may circulate by gravity of electric pump.


What are the different types of solar powered water heaters?

As in many things, solar power water heaters come in passive and active types. The passive style has no moving parts. The warmer liquid rises and is replaced by cooler fluid. In this way heat is transferred from the solar collector to the heat exchanger.

In an active solar water heater an electric pump is used to move the transfer fluid around. Both systems are usually pressurized to 1 bar and can be protected by mixed with a small percentage of food-grade ‘anti-freeze.’

Solar water heating system diagram

active solar thermal heating diagram

Types of solar water heater collectors


Flat Plate Collectors

There are many forms of flat plate collectors, some more sophisticated than others, but they all basically contain some kind of tubular framework or manifold soldered or welded together and containing water-based heat transfer fluid.

The tubing is constructed from thin-walled copper or other metal having rapid heat transfer characteristics. (For large pool heating installations where area is not so critical, plastic  piping can be used to gather and transfer heat.)

The tubular manifold is painted black , laid on a bed of insulating fiberglass wool and covered in glass or other glazing material.

Flat plate solar collector efficiency

Flat plate efficiency is a measure of how much heat it transfers as usable energy. This value can vary between 30% to 70% and depends on:

  • collector build quality
  • materials used
  • local irradiance

Evacuated tube collectors

I have personally installed both types of solar collector systems professionally and think that the evacuated tubes are by far the most efficient and easiest to install/maintain.

Evacuated tube thermal solar collectors in action

“Balzac Fresh Food Distribution Center – Solar/Wind” by Walmart Corporate is licensed under CC BY 2.0

Evacuated tube solar collector efficiency

Evacuated tube collectors can be up to 90% efficient. How do they do this? An evacuated tube is suspended inside a thicker outer tube which serves to focus and concentrate the sun’s rays.

A small amount of fluid is contained inside the inner tube which boils rapidly due to the reduced pressure. The vapor rises to the top of the tubes, which are mounted at an angle, and transfer their heat to a circulating fluid in a manifold running across the top of the collector.

evacuated vacuum tube solar collector

Compare Evacuated Tube Collectors With Flat Plate Collectors

Evacuated Tube Collector

Flat Plate Collector

Generates heat quickly

Slower heat generation

Efficiency up to 90%

Efficiency up to 70%

Low heat loss (evacuated tubes)

Considerable heat loss (copper pipes)

Works when very cold

Needs anti-freeze

No internal scaling of pipes

Needs de-scaling periodically

Cylindrical tubes collect sun at all angles

Flat collector absorbs sun's rays different rates

Heat exchanger not needed

Need heat exchanger

Hot water available all year round

Realistic usage 300 days per year

Low maintenance

High maintenance

Glass tubes easy to replace if broken

Whole glass top needs replacing


DIY Solar Water Heating

This is an area where the dedicated DIY practitioner or off-gridder can really ‘go to town’. There are many plans available and hundreds of Youtube videos describing how to build thermal solar heaters and I’ll be posting detailed plans in the future, with particular attention to the ideas below i.e. combining PV with solar thermal to get the maximum energy transfer from the sun.

Hybrid PV Solar Thermal Collector

An exciting (not-so-new) development is the hybrid solar collector, which makes use of electricity conversion in the form of PV and thermal transfer of the sun’s energy. The video below explains it:

Solar PV is a great technology that has grown leaps and bounds particularly over the last two decades but its efficiency remains around 20 percent, even in premium panels, so only 20 percent of the energy intercepted by the solar panel is converted into electricity. The rest of the 80 percent is lost.

Now this may not be important for people that have large roof areas for installing solar panels but for people with limited roof space, like in an urban setting, that 80 percent lost energy means a lot more. Even a cursory look at the solar PV technology tells us that it is approaching a point of maturity, although technology development is adding extra power to the panels, but only in small increments.

In recent years we have seen cut-cell and bi-facial solar technology has improved the gain of panels but we are now coming to a point where in order to squeeze more out of the pv panels we have to look either towards the prohibitively expensive multi-junction solar cells used in satellites or bank on perovskite cells which has the potential to go further than crystalline silicon cells.

But at present it remains unstable and degrades quickly, so is there a way to make more hay out of solar irradiance? Yes there is. It is by using a solar PVT panel, or in other words, a photovoltaic thermal hybrid collector.

As the name suggests it is a combination of solar thermal collector and photovoltaic cells that maximize the solar gain. With the simplest of methods it provides both electricity and heat as an output and the best thing about it is that the two technologies complement each other and work symbiotically.

If we look at solar thermal collectors on their own we find that they are very efficient devices. In fact the evacuated tube collector has been recorded with energy efficiency of over 90 percent, meaning they can convert nearly all of the incident sunlight into heat.

Solar PV on the other hand can only work with photons in the sunlight of particular energy levels and the rest of the photons simply pass through and get absorbed by the back layer and end up producing heat in the cells which is undesirable.

In fact, this generated heat reduces the output of the solar cells. The thermal coefficient of the PV panel a value that is provided with its specification sheet tells us precisely the drop in performance of the panel with rising temperature. In desert climates the PV panel temperatures are known to reach above 70 degree centigrade.

To cool the panels down options cooling jackets are used. In solar PVT panels the photovoltaic cells are placed on top of a solar thermal collector. The excess heat that builds up is removed by water running through the thermal collector.

It has been claimed that hybrid panels can have an efficiency as high as 85% and can generate four times the energy produced from the same surface area for only 25 increase in cost.

In Europe and many other cold climate countries, over 52 percent of the total energy used goes into space and water heating heat remains the biggest energy used. User studies have shown that active heat removal systems can improve the PV panel life from 30 years to 50 years, so heat removal not only improves the instantaneous performance of the solar PV cells but also adds longevity to their life.

20% higher annual output of electricity has been reported for PV alone in a PVT system compared to a non-PVT system. One may ask that if this is the case, then what is the hitch. Why is this technology not so prevalent? Why is it that only a small number of companies are producing it?

Well it turns out that PVT technology is not simple plug and play as PV technology is. Furthermore, in summer time one can end up with large quantities of water that is 35 to 40 degree centigrade in temperature and has nowhere to go.

For this reason people with swimming pools are the ones that are opting to install them for now but there is a way around it. The excess heat can be dumped by passing the hot water through outdoor convectors when it’s not needed.

The fact remains that when we are using both electric and thermal outputs of the PVT. We are essentially saving double the amount of money. In many places electricity prices are three times the price of gas and this is exactly the ratio of output you get from a PVT solar panel.

Related Questions

How do active solar collectors work?

All solar collectors have a circulating fluid that transfers heat from the sun to a heat exchanger. Active solar collectors use an electric or other pump to move the fluid around the circuit.

How do passive solar collectors work?

Passive solar collectors contain a fluid to transfer the sun’s heat to a heat exchanger. In a passive design the fluid moves by thermal action. As it heats up it rises to the top of the angled collector and is replaced by cooler liquid.

Why is glass used to cover solar collectors?

Glass has a long life and does not tarnish. Light and heat energy pass through easily but lower frequency heat cannot return so easily. This means that heat remains trapped inside the collector and has more chance of being transferred to the collector heat transfer fluid.

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