Photovoltaic Water Heater: The Build

Photovoltaic Water Heater System

Schematic of the water heater system. The lower heater element runs off of solar power. The top element runs off household power.

On the supply side there are two 330W panels connected in series using MC4 connectors, which are standard for the solar world. Solar panels are live whenever they are lit and so it is good to be able to safely unclip the panels should you need to disconnect the power from the rest of the system.

Photovoltaic Panels

Photovoltaic panels mounted on adjustable brackets to the roof.

The panels are mounted to an adjustable framework that can be tilted up or down to better match the solar angles during the year. At the moment the plan is to adjust the angle four times a year to correspond to the best angles for the seasons. In theory I could also connect a linear actuator and have it automatically adjust perfectly to match angles throughout the year.

Solar Panel Breaker Box

The service box where the solar power enters the building envelope.

From the panels, wiring runs through a section of flexible conduit and then through non-metalic conduit hidden in the eaves of the house and down into a service panel located just inside the house. The panel contains a suitably sized breaker that will trip if overloaded and also acts as a shutoff for the power entering the house. The shunt for my metering unit is also inside the panel.

The box itself was bought empty from a salvage yard and then assembled using rails and other components salvaged from a household breaker box. Back when I was running an electrical service to my garage, my supplier had some panels containing suitably sized main breakers for the main panel shutoff at a price that was close to that of the breaker alone. Rather than just buy the breaker, I bought a whole extra panel and put the rest in my parts shed. Since then, I have managed to scavange a sum of parts off of it that were worth much more than the original price discrepancy.

Attached to the panel are the various meters. At the moment I have a little multimeter unit that shows me voltage, current, power and energy. I also have an hour meter that is connected to the main solar circuit and shows the overall hours the system has been running at a high enough power to run the meter itself. I also have on order two alternating current hour meters that will be used to show the run times of the upper and lower thermostats of the water heater.

Wiring up the inside of the panel.

Wiring up the inside of the panel.

The last item connected to the panel is the relay. A relay is basically a switch that is used to control one electrical circuit using the state of a second electrical circuit. In this case, it is set up to use the 240V AC output from the lower thermostat of the water heater as a signal to control the circuit connecting the solar panels to the lower water heater element. When the lower thermostat of the water heater is active and would normally be sending 240V AC power to the lower heating element, it instead closes the solar circuit and allows whatever power is being generated by the panels at that time to heat up the lower element.

Running between the panel and the water heater is a wire harness including a ground, the two heavy solar power conductors, and three wires from the thermostats – a “hot” connected to each of the thermostats as well as a ground. At the moment, the hot wire from the top thermostat is just capped off but when my hour meters arrive it will be connected to one of those to keep track of how long the upper element has been running.

Attaching an electrical box to the top of the heater.

A box on top of the heater provides room to safely connect all of the wiring. A metalic conduit is used to run the large solar wires to the bottom element of the heater.

With all of these extra wires, I added a box to the top of my heater to take in the wiring from the solar system as well as from the mains and connect them with ample space to the wiring coming from the water heater.

Although I was able to run my 14 gauge wiring for the low amperage 240V signals from the thermostats through the body of the water heater itself, the physical constrains of the bracing inside the heater as well as concerns over heat building up and derating the larger gauge DC wiring had me running it in a metalic conduit outside the water heater down to the lower element.

From there, the two solar connectors attach to the lower heating element using lugs. The element itself is a 120V 1500W model that has the right resistance of the system – I’ll cover exactly what I mean by that in the next entry. Now, there is no inherit difference between DC and AC heating elements themselves, but the connectors on the element are definitely meant for the much smaller wiring used in AC systems. In order to create a safe connection, I used the existing screws to mount lugs and then the lugs to connect the wiring. Installing the replacement element is as simple as draining the water heater and using a heater element wrench to remove and replace the element.

The rest of the electrical inside the water heater has been left intact. The emergency cut-off, top thermostat and heating element as well as the bottom thermostat are all connected as they were. The only difference is that the output from the lower thermostat now gets fed back into the coil circuit of the relay rather than directly to the lower heating element.

Completed water heater

Completed water heater, mixing valve installed.
All that remained was the re-install the insulation and covers and then insulate the mixing valve and hot water piping.

The last part of the build was the addition of a mixing valve to the output from the water heater. In order to take advantage of sunny days, the lower thermostat is cranked to its maximum position – over 150 degrees farenheit. This allows the solar circuit to heat up the water to well above the ideal output temperature of the tank. This is great from a gains standpoint but introduces a problem, both in terms of convenience and safety. If left unchecked, the result would be that after a few days of full sun the temperature coming out of the tap might be dangerously hot compared to the same faucet setting after several grey days. This could lead to scalding at worst and even at best would be inconvenient.

The solution is the use of a mixing valve that I already had on hand. It connects to the hot water line coming out of the water heater, the hot water line heading to the rest of the house and a cold water line branching off of the line coming into the water heater. You set the desired temperature on the mixing valve and if the temperautre coming in from the water heater exceeds this setting, it will temper it with cold water to keep the outgoing water at the desired final temperature.

It may seem a waste to cool down water that you’ve just heated up, but the upshot of this is that you end up using less of the very hot water than you would a milder temperature. Theoretically, you are heating the same final volume of water to the same final temperature and so the heat extracted from the tank should be the same. While real-world thermodynamics are always somewhat more complex than that, the principle is roughly correct.

Next up: Math and Design. Stay tuned (and feel free to subscribe to get email notifications).

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