Can a standard off-the-shelf tank style electric hot water heater can make a dump load?
Electric tank water heaters are an excellent choice to make a dump load for your solar, wind or micro hydro power system as they are easy to get, cheap, already insulated and have holes to mount low voltage water heating elements.
You can usually find a pretty good looking and quality electric hot water heater just about anywhere in the world that can be used either as:
- a pre-heater for your main hot water heater if you are dumping power from a normal sized solar system or
- your main hot water heater if you have a large, consistent supply of electricity such as a good quality water turbine or high power wind turbine.
Even though an off-the-shelf tank style hot water heater can make a good dump or diversion load there are a few concerns such as:
- The water heating elements in a standard tank heater are made for high voltage (either 120 or 240 volts)
- The thermostats in a standard electric water heater are not made to handle DC and will either arc and stop working or catch fire
- When the tank of water becomes too hot, where do you send the excess electricity?
We will answer the above questions in a few minutes but let’s discuss the most common question I get about using tank style water heaters for a dump load/water heater.
The first question I get when folks decide to go this route is:
Why can’t I just use the output of the inverter to dump electricity (when the batteries are full) into the existing water heating elements using the factory thermostats?
While it is possible, it is likely not a good idea.
The first problem is that if there is ever a problem with your inverter, your dump load is non functional and it only takes one really long overcharge for your battery bank to be damaged.
The second problem is hot water elements are usually high wattage 2500-5000 watts which could be enough to consume all the electricity your inverter(s) can produce, depending on the size of your system. If you use your entire inverter’s capacity, there will be nothing left over for your loads like refrigerators, freezers, computers, dishwashers, and all the other goodies we have grown accustomed to.
The third problem with using AC from your inverter to operate your dump load is your will need to set your system up to turn the dump loads on at a particular voltage (your bulk voltage) and off at a particular voltage (something lower than your bulk voltage). This is how old solar charge controllers used to operate. They had a hysteresis (difference) of a few volts and would turn the solar array off at about 14.4 volts and back on at roughly 13.2 volts in a 12 volt nominal system. There are two problems with this:
a) The voltage is always changing between the on and off voltage and we have learned that batteries will last much longer if the bulk voltage is held steady with pulse width modulation (PWM) rather than going up and down constantly.
b) Your 2500-5000 watt element(s) are a huge load on a battery bank unless it is huge making the heating elements cycle on and off sometimes many times per minute. This is very hard on your inverter and will make your lights blink on and off as well as make your refrigerator and freezer compressors unhappy.
How can I keep the battery bulk and absorption voltages steady so my batteries will be happy and last longer?
This is where PWM charge controllers came in. PWM stands for pulse width modulation meaning the load (or solar array) is switched on and off several times per second in trying to hold the battery voltage as steady as possible. PWM is much easier to understand than most people think.
If your batteries only need 100 watts to hold the voltage at the bulk voltage but your solar array is making 1000 watts, PWM will make sure the circuit (solar array to batteries) is connected 10% of the time and open circuited 90% of the time just like the first of the three PWM examples in the diagram below.
EXAMPLE #1 Your solar array produces 1000 watts but your batteries only need 500 watts to hold the bulk voltage. The PWM solar controller simply turns the solar off 50% of the time and on 50% of the time. 50% of 1000 watts is 500 watts. The only issue with PWM is it is fast. It does its switching hundreds or thousands of times per second.
EXAMPLE #2 Imagine you want to dump 1000 watts of electricity but your dump load is a 3000 watt dump load. A PWM dump controller or 12 volt relay driver (built in to most quality inverters and charge controllers) turns the dump load (or solid state relay) on 1/3 of the time and off 2/3 of the time. 1/3 of 3000 watts is 1000 watts. Of course it does this hundreds or thousands of times per second.
The downside of pulse width modulation:
Standard relays (coil type) cannot handle that type of switching. They will be destroyed. Only solid state relays (SSRs) can handle this type of rapid on/off switching. Another issue is PWM controllers will cause noise in your phone lines or other sensitive electronics.
So the question remains…
How do I use a standard hot water heater (tank style) as a dump load that will make free hot water?
The best method is described below. There are other methods but this is the most reliable and accurate.
STEP 1 Purchase the following:
* Two (or one if you are only using one half of the capacity) low voltage 12/24/48 volts DC water heating elements large enough (enough wattage) to dump your excess power.
They are usually available in the following configurations:
- 12 volt system: 450 watt and 900 watt X 2 elements = 900 or 1800 watts maximum at 15 volts DC
- 24 volt system: 720 watt, 900 watt and 1440 watt X 2 elements = 1440 or 1800 or 2880 watts maximum at 30 volts DC
- 48 volt system: 1440 watt X 2 elements = 2880 watts maximum at 60 volts DC
You will notice the voltages above are somewhat higher than the nominal battery voltages. This is because you typically dump excess power at approximately 14.4-15.0 for a 12 volt DC system, 28.8-30.0 for a 24 volt system and 57.6-60.0 for a 48 volt system.
Adapters to convert a threaded DC element to a square unit with four bolt holes if necessary.
All low voltage elements used for diversion loads have a 1″ male national pipe thread (MPT) and sometimes they even have a gasket as well. Most of the time there is no gasket.
Although many hot water heaters have a female pipe thread (FPT), some have a square hole with a threaded bolt hole in each corner.
If you have the square hole type you will need one adapter, gasket and bolts per low voltage water heating element.
* Two solid state relays (SSRs) that are rated for DC at the amperage (depending on the size of your element(s)) and voltage (more than 15,30,60 volts DC) you will be using. You will need one SSR per heating element.
The SSRs also must be able to be triggered (turned on) at 12 volts as we will be using one of the 12 volt relay drivers already installed in your charge controller or inverter.
You can usually get DC SSRs that are good for 40 or 60 amps at up to 160 or 220 VDC that are triggered by 3-24 volts on places like eBay.
* A charge controller or inverter that has a programmable 12 volt relay driver such as one made by:
- Outback Power (all of their inverters and charge controllers have one),
- Magnum Energy (all of their inverters have one or two)
- Schneider Electric (all of their inverters and charge controllers have one),
- Morningstar Corp and a few others. Some call them voltage controlled relay drivers or voltage controlled relays or voltage controlled switches.
- Another option is the Morningstar RD relay driver. (It has four different channels or relay drivers that can be turned on and off at certain voltages and even temperatures. They are even programmable through a serial cable from your laptop or desktop computer.) If your equipment already has relay driver’s you will not need the Morningstar RD.
* A dump/diversion controller such as Morningstar’s TS-45, TS-60 or Schneider Electric’s C-40 or C-60 for a secondary dumping system for when the water is too hot in the water tank or the original diversion system malfunctions.
* Use another of the programmable relay driver’s (in your existing equipment) in conjunction with a solid state relay or standard relay.
* A secondary dump load such as an air heater/resistor. You may have to use multiple units and place them in a metal vented electrical box or purchase a manufactured unit.
Now before you start to think this is going to be a massive and confusing project, relax and only worry about one item at a time.
For all the next steps listed below we will include wiring diagrams and explain things as clearly as we can.
STEP 2 Remove the high voltage and high wattage AC elements and replace them with DC lower wattage elements.
STEP 3 Connect the negative lead from your battery/DC breaker panel to one side of each heating element.
STEP 4 Mount one solid state relay (SSR) to the outside of your hot water tank for each element you will be using. SSR’s can get hot so it is a must that each SSR be mounted to a large piece of aluminum or heat sink.
I apologize this article is not yet complete. I clicked the publish button by mistake. I will finish as soon as possible. Thank you for your understanding….