Photovoltaic DC Water Heating

We Filed our Provisional Patent on Sept. 13, 2016

We Filed an updated Provisional Patent on Feb. 8, 2017 (which has the most comprehensive claims and drawings)

We Filed a Patent Application Sept. 13, 2017


The portion of our electricity generated by renewable energy will continue to experience rapid growth. Most of these sources of electricity are variable – meaning we can’t turn them on and off, and they change in time. In fact for wind power, it is difficult to turn off the generation of the wind towers, so this electricity needs to be used at the same rate it is generated with the following niche market for add on electric water heating:

  • Consumers find that electricity from renewable generation is sometimes free. For instance in Texas, electricity is free at night because of surplus wind power and otherwise low demand.
  • Homes with grid-connected PV system may produce more electricity annually than they consume. Through the agreement with the utilities, they would then be entitled to consume more electricity free of charge, and could heat water with this extra electricity.
  • Additionally, for people who have solar panels not connected to the grid, any electricity that is not being used at the moment of generation is free. This electricity can be stored in batteries, but batteries are expensive. Thus this surplus electricity could displace natural gas in heating water (for instance). With the continued decrease in PV, an increasing number of users will likely choose to not connect to the grid.
  • Electrical utilities are challenged to match the load to the supply. Traditionally, utilities have matched load by turning on and off disatchable generators, which is control we call supply side management. However, the situation is much more complicated when the renewable electricity is also variable and makes up a larger and larger portion of electricity production. It would be very nice if the electrical utilities could turn on and off dispatchable loads in order to make the load match supply (for demand side management or demand response). Thermal loads are not as time sensitive as other loads, for instance, you don’t care if you hot water is heated at midnight or 3:00 AM, as long as it’s hot when you take a shower. This using electricity to heat water (for instance) represents a dispatchable thermal load that can be used to smooth the grid. It may be possible to have the electricity system operators control these electric heaters in order to stabilize grid power.

So, we figured out how to add an electric immersion heater to any water tank.

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The above graph from Green Tech Media illustrates the growth of the PV market

Design Process

The original application for this device was to heat water at The Grange in SLO. Because of this we used the specifications from The Grange for our design process. The Grange currently has one 40 gallon water heater tank that runs off of natural gas. We needed to create a system to heat the water in the natural gas tank using DC power. Another specification for The Grange is the need to increase their capacity of hot water, because currently 40 gallons is not quite meeting their needs. For that reason, as well as the fact that water is a good way to store thermal energy we decided to heat the water to about 90C. This is much hotter than most residential water heaters that heat up to about 50C. We cannot allow 90C water to flow through the pipes and potentially out the faucets at The Grange, so we will need to implement a hot water mixing valve at the tank output-but this will be later down the road.

The Design process including our multiple ideas can be found here.

Our final design makes use of any port of a water tank, in most cases the drain port. Using a swivel adapter, “T” joint and threaded nipple commonly found at hardware stores an electric heating element can be inserted into the port of any water heater, natural gas or electric. The “T” joint enables the drain spigot to be used so the function is not lost. In our example at Pete’s house we used a separate swivel adapter and “T” joint. But ideally a piece is manufactured that includes a swivel and “T” in one piece that is shorter so the heating element can be immersed in the tank as much as possible.

Heating element in swivel with drain and 4th open port option

The heating element can be bought at any hardware store. For most applications the port in the water tank is fit for 3/4″ NPT. Using a factory made heating elements results in a problem. The element will not fit into the drain unless some retrofitting is done. We found a way to bend the element to allow insertion into the swivel adapter and thus the water tank. The complete assembly of swivel and element costs no more than $30.

We bent the element using a vice applying pressure to the element in the direction of the arrows. The unique part of our set up was that we put a circular metal piece in the inner radius of the element for the element to bend around and we used a hard composite board (grey board on the left) to push on the top, outside radius of the element. This solved our previous problems with the element cracking at the top outside radius.


Left element is bend by us as described above. Right element is unmodified.

A critical component of this design is regulating when the heater is on/off. One way the user can regulate this is using a thermostat. When using renewable energy sources that vary, it is beneficial to allow the water to get very hot to store the available energy thermally. A standard thermostat found on most electric water heater tanks can be used. The temperature the thermostat switches at can be set to be a higher temperature than the ratings on the thermostat. For example the thermostat in the figure below looks like it only goes up to 150F adjusted by twisting the central dial with a screwdriver. We found that the dial can be twisted farther than the rating for 150F and can be set to be triggered by a higher temperature. Some trial and error is necessary to know how far the dial must be turned to correspond with 90C. With the thermostat set to such a high temperature a mixing valve is needed at the output of the tank to bring the water to a safe temperature to go through pipes and for people to use. In total, the thermostat and wiring necessary for installation will cost close to $25. As an added feature, utilizing two thermostats (a default and an add-on) can increase energy storage. Setting the add-on thermostat set point higher than the default will provide better management of energy demand based on the amount of electricity available for use.

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Electric water tank thermostat

To attach the thermostat to the side of the internal tank of the water heater a Dremel can be used to cut into the outside metal tank. We found that only three sides need to be cut away and the metal can be bend up as a flap (see figure). The insulation from the tank needs to be removed and thermal grease can be added before the thermostat is placed in the hole and touching the inside tank to ensure a thermal connection. Insulation then needs to be added back around the thermostat after it is hooked up in series with the heating element and the power source, and then the metal flap can be bent down.

Flap with yellow insulation in side of tank can be seen. The thermostat is under the insulation connected by wires.

Water tank with Add-On Electric Water Heater installed along with thermostat

Add-On Electric Water Heater in electric water tank drain port hooked up in series with thermostat. The element directly under the thermostat is from the original use of the tank and can remain hooked up to the grid or a different power source.

Example Power Calculations for our example system that uses SunPower 425W PV panels.
Rough cost analysis

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Diana Swanson

Diana Swanson is a second year Physics major at Cal Poly, SLO. Diana is excited about this project because of the direct application and the hands on work, applying physics and engineering concepts to a project that will allow people and places like The Grange to reduce there dependency on grid power. She is from West Linn, Oregon and enjoys spending time outdoors hiking, camping, kayaking, running, playing ultimate frisbee and soccer. Diana is pursing environmental studies and astronomy minors and is also doing astronomy research with the RECON group at Cal Poly looking at TNOs.

John D’Ambrosio

John D’Ambrosio is a fourth year Mechanical Engineering student at Cal Poly, SLO. Born in Las Vegas, Nevada but raised in Thousand Oaks, California, John enjoys working out at the gym, playing basketball, making music, and going to the beach. During the school year, John is the Vice-Chair of Cal Poly’s ASI Board of Directors, with responsibilities including facilitating board meetings, representing the student body, and working directly with administration. His motivation for joining The Grange project stems from his drive to work for a higher purpose and reduce our carbon footprint. John seeks a career that incorporates sustainability and alternative energy with mechanical engineering.