It all starts with the storage tank.



The storage tank is the center of every solar hot water system. Whether the heat source is Solar thermal, a heat pump or a wood fire, there is always a storage tank needed.

We focus on making the best quality storage tank with the most versatile design and then build the renewable hot water system around the tank.


DUPLEX Stainless Steel.

The Exegi hot water storage tanks are made from 2205 Duplex stainless steel.

Why is Duplex the best material for hot water storage?

The short answer is it is stronger and more rust resistant than the most common stainless steels used to make hot water storage tanks.

Duplex steel was designed in Sweden in the 1930’s but is only now becoming popular as the manufacturing process has improved significantly.

Understanding stainless steels and which type is best for a specific application is very difficult.

Below are the key benefits to DUPLEX stainless steel.

  • Twice as strong as 316L

  • Thinner sheets can be used saving materials and also transportation weights.

  • High weldability

  • The welding process for Duplex Stainless Steel is very specific. The main engineers and welders that make our tanks have been trained in Norway at Outokumpu. All the Duplex steel used in our products comes from Outokumpu.

  • Highly resistant to Stress corrosion cracking, pitting corrosion and Crevice corrosion.

  • Cracking corrosion is one of the main problems for stainless steel tanks


Why is our design better?

  • Versatile ports to suit a wide range of applications.

  • Tank feet to stop heat loss and protect the base of the outer tank from corrosion.

  • Drain port to enable proper servicing increasing the longevity of the tank.

  • There are many others.

    We listen to our customers and have adjusted our design over the years to be, in our opinion,




Evacuated tubes solar hot water

Renewable solar hot water can be achieved in many ways.

We all want to save money and lessen our impact and focusing on generating hot water is a great place to start.

There is an Exegi Duplex Stainless steel tank for every application that is compatible with and designed for the following renewable heat sources:

  • Solar thermal. Evacuated tubes or Flat plate collectors.

  • Heat Pumps.

  • Wood fire/wetback heated water.

  • PV powered Electric elements/PV diverter.



Evacuated tubes were invented to minimize heat loss in solar hot water heating typically experienced with Flat plate solar collectors. The construction of a flat plate collector uses a piece of glass to allow the heat into the collector but struggles to stop heat leaving the collector as glass is not a good insulator.

Evacuated tubes use 2 layers of glass with a vacuum in between the 2 layers. The outer glass is made of borosilicate glass which is very pure and does not contain any metal or materials that can become hot. The glass has very low thermal conductivity meaning it does not get hot therefor all the heat in the form of radiation that passes through the glass, does not get absorbed by the outer glass and is allowed to pass through it to the inner glass. The inner glass has 3 coatings of heat absorption and transfer material that takes the radiation and becomes hot.  The heat is then transferred using conduction to the water or liquid that is being heated. Once the heat is in between the 2 layers of glass, the vacuum does not let it out as a vacuum is an amazing insulator.  Sounds confusing? Well yes but the main points are the heat can get into the tube but not out again thus given an evacuated tube an efficiency of above 92%. Meaning that 92% of the heat that hits the tube goes in, but does not come out again.

Try touching an evacuated tube on the outer glass that is in the sun, it will be almost exactly the same temperature as the air. PLEASE DO NOT TOUCH THE INNER GLASS! The inner glass can reach temperatures of over 250 Deg C in a matter of minuets.  Yes, 250 Deg C! Even on a 15 Deg day with clear skies.  The air temp does not effect the efficiency of the tube because of the vacuume. Its all about the power of the sunlight.

 As with the flate plate collectors, the heat is transfered to the stored water once hot for later use. 

The process of transferring the heat from the inner tube to the storage tank can be done in 2 ways.

Thermosyphon or pumped.

Using a thermosyphon to transfer the heat to the tank needs 1 thing. The tank must be higher than the solar collector. Heat rises. Hot air rises and so does hot water as it is lighter than cold water.  It is lighter because hot water expands when it is heated and therfor is less dense.

If the storage tank is lower than the solar collector the hot water needs to be pumped down to the storage tank using a pump and a temperature differential controller.



Over 400% efficient!

We only sell CO2 Heat Pumps.

Why? Because they are the best Heat pumps available for heating water and work best in most climates.
With a COP (coefficient of performance) of 4.5 a CO2 air source Heat Pump will generate 4.5 times the amount of heat compared to the energy input.

CO2 Heat pump with a Duplex Stainless Steel tank coming soon!

How a CO2 Heat pump works.

A heat pump uses the energy in the ambient air (Air heated by the sun) and transfers that heat to water using a heat exchanger.  Lets say the air temp is 15 deg C. So how does 15 Deg air heat water to a temperature that can be used in the house?  Pressure! And lots of it.

The heat pump has a fan that blows the air past a radiator (like the one in your car) and the radiator transfers the heat in the air to a refrigerant gas.  The gas is now 15 Deg.  The gas is then pumped through a compressor and pressurized. As the gas is pressurized, the temperature in the pressurized gas increases. The gas is pressurized to a point where the temperature of the gas is 65 Deg C! The 65 Deg C heat is then transferred to water using a heat exchange. The hot water is then transferred to the hot water storage tank for later use.

Using a refrigerant gas and a compressor it is possible to multiply the energy input by a factor a 5. Ie, an efficient heat pump can produce 5 times more energy (in the form of heat) than the energy used (electricity) to run the machine. Sounds like magic?  NOPE. Just a smart way to use electricity if what you need is heat or hot water.  Your fridge uses this same principle but in reverse. It takes heat from the food and expels it out of the fridge through the radiator in the back.  Food/ external heat source heats the fridge, heat is transferred to a gas, gas is pressurized and therefore increasing the temperature differential compared to the ambient air,  heat is transferred to the ambient air.  Everything is trying to be the same temperature all the time. Heat will transfer freely.



Hang on, what has a wood fire got to do with Solar hot water?

How do trees get energy to grow? From the sun!

You can use a wood fire to not only heat a space but also heat water using a heat exchange. 

Burning wood releases the stored potential energy stored in the tree and converts it to Kinetic energy (heat) using combustion. 

The heat can be transferred to water using a heat exchanger. Heat on 1 side, water on the other. The hot water can then be stored in a hot water storage tank for later use.

There are many different types of wood boilers including domestic wood fires with a wet back heat exchange right through to large external wood boilers that can heat your whole house using hydronic heating and your tap hot water!

Using a wood fire to heat you water in winter matches up beautifully with solar thermal or a heat pump as the heat source for you hot water in summer.

When its cold and there is no solar radiation, you have the wood fire on. When its hot, no wood fire but you have sun. Simples.


Contact us.

We can help you if what you need is energy effecient and sustainable hot water. If we do not answer the phone, please leave a message or send an email.

Contact us by email

Contact us by phone.

03 9397 3993

Name *

Warehouse and office hours.


Monday to Friday

or by appointment.


4/3 Techno Park Drive
Williamstown, Victoria 3016, Australia


Exegi Duplex stainless steel tank range and technical details.

Custom made tanks are also available.