Introduction.

In this article, I will explain how to design the perfect water heating system using renewable energy. The ideal system would consist of a three-stage system. Stage one is heated by solar PV, stage two by heat pump, and stage 3 by gas. This is the most cost-effective way to make use of renewable energy and ensure there is always hot water.

Why a three-stage system?

The first stage will be heated with solar power directly, so, solar thermal water heating. This water heating can obviously only happen when the sun is shining. Also, when the water heating starts early morning there needs to be cold water in the storage tanks for the solar heating to be able to heat the water. If the water is already hot in the storage tanks then the solar water heater can not do any work as the water is already on temperature. But also, most people shower in the morning and need to have hot water in the morning. So, a mismatch between what the solar water heating system needs and what the people need. For this reason, two hot water storage tanks are required. The first tank is fed by cold water and is cold in the morning. The second tank is fed by the warm water from the first tank. The second tank is kept always at the required temperature 24 hours per day. The second tank is heated by a heat pump.

Stage 1 explained.

Us a 2000 L insulated tank for the warm water. The tank must be able to withstand up to 60 degrees Celsius. Stage one could be on the ground level as no pressure is required yet. Stage 2 can be installed on a tank stand 5 meters high. A DC element can be installed with a thermostat to switch off the current once the water has reached 60 degrees Celsius. The DC element is connected to the output of an MPPT. A 17kW DC element is needed.

Solar PV panels or Solar Thermal panels?

With the price of solar PV panels getting really cheap, it might be more cost-effective to use solar PV panels instead of Solar thermal panels. The use of Solar PV panels has certain advantages and disadvantages compared to solar thermal panels.

Advantages of solar PV over Solar thermal panels.

1. Panels can be installed far away from the solar water tank on any suitable roof.
2. No pumps to install or to maintain.
3. No risk of frost that can break pipes and solar thermal panels

Disadvantages.

1. Solar PV panels are not as efficient per square meter as solar thermal, solar thermal is about 80% efficient, and solar PV is about 25%. This means one needs an area that is about four times larger for PV compared to thermal.

How much Solar PV is required.

2000L requires 85kWh of energy to heat up from starting temp to 55C. If we assume the specific energy yield in Portugal is 5 full sun hours. The 85 / 5 = 17. 17 kW of solar PV is needed. If we use 500 Watt solar panels, that means we need 32 x 500 Watt solar panels.

Stage 2 explained

So the pre-heated water will come from the top of tank 1 to the bottom of tank 2. Tank 2 one always want to keep between 45 degrees Celsius and 60 degrees Celsius. 45 degrees is still hot enough to take a comfortable shower. One always want to have hot water in tank 2. In tank one the water temperature must be cold in the mornings for the solar panels to be able to work. Tank 2 will be heated with a heat pump. The heat pump will switch on at 45 degrees Celsius and switch off at 60 degrees Celsius. It will also only work when the battery state of charge is above 80 percent. So it will never drain the battery to bellow 80 percent. This will leave enough battery power for other devices, and also essentially use any excess solar power. This will make bettery use of the solar panels connected to the batteries and ensure that it rarely happens that the solar PV system connected to the inverter and battery always produces energy and is not idle because the batteries are full and there are no loads currently using power.

Stage 3 explained.

Stage 3 is a gas water heater. If the water from stage 2 is already a the right temperature then the gas burner will not turn on and not be used. But if for some reason the water is not hot enough yet, then the gas burner will turn on. So, essentially a backup heat source.

Option and alternative to gas heating.

Even though gas is easy to use and convenient, it is a fossil fuel and expensive. A great alternative to that is wood pallets. This could be especially cheap if one has access to a lot of wood and can manufacture the wood pallets yourself.

Hot water system diagram

Click here to download a diagram of the system

Further Reading