As the price of photovoltaic modules (solar modules, solar panels) continues to drop there are many discussions about using solar electric modules to heat water. No more leaks, messy and complicated plumbing, heat exchangers, controllers to fail, pumps to maintain and fluids to freeze.
Solar hot water has been plagued with leaks, freezing (or poisonous antifreeze), air in the pipes, controller and pump failures just to mention a few of the difficulties we have had over the past few decades.
However it was the only way to make hot water with the sun. Just a decade ago solar electric modules cost over $10 per watt. Solar thermal was much less expensive per kW or BTU.
Our goal is not to bash solar thermal hot water as it serves a purpose, has performed well for many including our own homestead, and produces more hot water per square meter (or square foot) than photovoltaic ever will.
We currently have a 20 tube evacuated solar hot water heater in conjunction with a heat exchanger, solar differential controller, circulating pump, solid state relay and it works incredibly well when it works. It provides about 50-60% of our hot water in the summer and 10-20% in the winter, which I consider pretty impressive when you consider we have a family of 6.
The photo below shows our solar hot water collector (with the pipe insulation removed) as we were looking for a leak on a cold winter’s day.
The problem is there always seems to be something wrong with the system…air gets in the system, the collector and/or piping leaks, the controller forgets to turn the pump on, the controller forgets to turn the pump off etc.
However, before go out to your local hardware store and buy an electric heating element and eBay for a solar panel there are a lot of challenges with solar PV water heating. There are correct ways to do it and incorrect ways to do it. If you do it wrong you might be lucky and get little to no hot water or you might be unlucky and burn your home down.
To our knowledge there are no “off the shelf” solar electric water heating systems approved for use in North America. There is a product in Europe called REFUSOL by Krannich Solar but it is CE marked for Europe but not UL or CSA approved. Maybe they are working on that but who knows.
Refusol has three separate solar DC inputs that will track the maximum power point voltage (MPPV) of your solar modules, combine it and feed it to the water heating element. We will talk more about this in a few minutes.
Next we are going to discuss the ways we can heat water with regular photovoltaic (PV) solar modules and electric water heating elements to make a usable amount of hot water for domestic use or home heating.
ALL OF THESE SYSTEMS REQUIRE SOME TYPE OF OVER-CURRENT PROTECTION IN THE FORM OF DC RATED FUSES OR DC RATED CIRCUIT BREAKERS.
DC OVER-CURRENT PROTECTION IS NOT OPTIONAL!!!
SOLAR PV WATER HEATING
As it stands right now there are at least eight ways we can use regular solar modules (photovoltaic) to heat water in a standard tank water heater. All of the systems below do not mention over-current protection but as mentioned above it is a must.
If there seems to be interest in heating water with photovoltaic modules, we will write articles (one article per method) for all eight methods complete with photos and drawings to explain each method thoroughly including where to install and how to size the over current protection.
1. Pick any solar module or group of modules and connect the power output to any water heating element. No thought, no controls and almost no chance of it working.
2. Connect a solar array directly to a hot water element (making sure you match the maximum power point voltage with the rated voltage of the water heating element) with no controls and no other components.
3. Connect a solar array to a maximum power point tracking (MPPT) charge controller with tiny 12-48 volt battery bank and setup a typical diversion/dump load system. Use the programmable relay driver (that almost all high quality MPPT charge controllers have built-in) to control a solid state relay to turn the low voltage (12-48 volts DC) heating element on and off. Once the tiny battery bank is full, the electricity will be diverted to your low voltage heating element.
You may also use off the shelf 120/240 volt AC water heating elements.
So far all the options we have mentioned don’t include any protection to prevent the water from boiling in our water tank.
This is very similar to the “good old days” when everyone had a range boiler behind their wood-stove.
A range boiler is a water tank that is usually placed behind, beside or above your wood-stove. Inside your wood-stove is a hot water front, metal tubing coil or U-shaped pipe or tubing to collect the heat from the burning fire. This hot water front or similar device is piped to the range boiler. When the fire is burning, heat is transferred from the hot water front to the range boiler and eventually the water will begin the boil.
In the good old days folks would simply open a hot water tap when they heard the water begin to boil. Remember this is a time before pressure relief valve’s were invented. In fact many people have been killed in the last century by using range boilers without some sort of pressure relief valve or system to dump the hot water before (or when) the water begins to boil and turn to steam.
The next few methods of solar electric water heating we discuss will all include some type of protection to stop the water from boiling. When water turns to steam it expands 1600 times. In 2015, with today’s technology, we would be crazy not to have some type of device or devices to prevent the water from turning into steam.
The following methods of heating water with solar electric modules include safety equipment to prevent the water in your system from becoming steam.
4. Connect a solar array directly to a hot water element making sure you match the maximum power point voltage with the rated voltage of the water heating element. Using a solid state relay (SSR) and a thermostat/aquastat we can set the system to automatically disconnect the solar array when the water reaches our preset temperature. We will also need a small power supply to turn the SSR on and off. Usually they are either controlled by either 3-32 volts DC or 120-240 volts AC.
WE CANNOT JUST USE THE “OFF THE SHELF” ELECTRIC THERMOSTAT INSTALLED IN ANY STANDARD ELECTRIC HOT WATER HEATER TO CONNECT AND DISCONNECT THE SOLAR ARRAY.
THE THERMOSTATS MOUNTED IN A WATER HEATER ARE RATED FOR 120 OR 240 VOLTS AC. THEY ARE NOT RATED FOR HIGH VOLTAGE DC.
IF YOU TRY TO SWITCH HIGH VOLTAGE DC WITH A STANDARD AC THERMOSTAT IT WILL EITHER BE DESTROYED OR CATCH FIRE!
5. Connect a solar array directly to a hot water element making sure you match the maximum power point voltage with the rated voltage of the water heating element. Use an “off-the-shelf” pressure/temperature relief valve to dump the hot water down the drain before it becomes steam. Wasteful in terms of water but simple and effective.
6. Connect a solar array directly to a hot water element making sure you match the maximum power point voltage with the rated voltage of the water heating element. Use a thermostat/aquastat and a solenoid valve to dump the hot water as the temperature of the water approach’s its boiling point. Of course we would also require a power supply to operate the solenoid valve.
7. Connect a solar array directly to a hot water element making sure you match the maximum power point voltage with the rated voltage of the water heating element. Using a thermostat/aquastat, a water pump, a solenoid valve, circulating pump, power source for solenoid valve and pump and a length of pipe buried underground (or concrete) we could dump the heat into the ground or concrete.
8. The last and most complicated (and most expensive) method of heating water with solar PV would be as follows:
- Connect solar array to a charge controller (hopefully MPPT), to a battery bank. With this system we do not need to try and match the solar module voltage with the rated voltage of the water heating elements. Instead we will match the solar module voltage with the requirements of the charge controller and battery bank.
- Connect the battery bank to a 120/240 V inverter.
- Connect the inverter output (120/240 volt AC) to an off-the-shelf 120/240 volt AC thermostat/aquastat an then the 120/240 volt heating element.
- Install an AC solid state relay (or standard relay suitable for AC) in the line between the inverter and the thermostat(s) and drive it using a relay driver from either your charge controller or your inverter. The relay driver in your charge controller or inverter is completely programmable to turn on at a specific voltage and off at a specific voltage. DO NOT USE THE PWM (PULSE WIDTH MODULATION) OPTION ON YOUR PROGRAMMABLE RELAY DRIVER. Only use the on/off setting with a hysteresis (difference of a few volts). PWM will destroy your inverter.
- Last but not least make sure the solar charge controller (the one mentioned in the first of these bullet points) is set to turn the solar array off when the water gets too hot. Do not want to overcharge the batteries when hot water is not longer needed.
This method has the advantage that you can use off the shelf elements, thermostats and over-current protection (fuses or breakers). You will essentially have built an off the grid solar system that can be used (or expanded) to operate other electrical appliances.
Let’s not forget we have been doing this for years in our solar off grid systems as dump (diversion) loads.
In a traditional off grid solar system the batteries will get full (hopefully). To prevent overcharge we currently deal with this by:
- Disconnecting the solar array(s) as needed to keep the voltage where it needs to be
- Dump or divert the excess energy into an air heater or other type of resistor or
- Dump the excess electricity into a hot water heating element.
Usually the excess electricity is dumped into a special low voltage DC water heating element. Check out Dump Load / Diversion Load Intro to learn about this process. You may also use 120/240 elements in certain circumstances.
So heating water with photovoltaic modules at 12-48 volts DC is nothing new but heating water with 120 or 240 volts DC is a whole new ballgame. You could easily produce an arc about 2 inches (5 cm) long with 240 volts DC.
Imagine what that would do to a AC fuse or circuit breaker?
High voltage DC is extremely dangerous and requires some special controls, over current devices and ingenuity.