Help with installing a solar water heater using solar electric modules
I’m a solar contractor in the Philadelphia region. I’ve installed traditional solar water heaters using flat plate collectors but a homeowner has asked me to install a solar water heater using solar electric modules and a heat pump. This is new to me, I did some basic online research and came across an article on this site written by Martin Holladay, “Solar Thermal is Dead.”
I found a company, Sun Bandit, that does not use a heat pump but rather uses energy from solar electric modules to heat an element in a tank. But the price for their package isn’t any cheaper than if I were to install a traditional flat plate solar water heater for my customer.
How do I design a solar electric water heating system? How many solar modules do I need? What other components do I need? Are there other manufacturers besides SunBandit that sell a prepackaged product?
I’d greatly appreciate any help you all can throw my way.
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Replies
Anil,
My article discussed the question of whether an investment in a solar thermal system was cost-effective. I concluded that it usually wasn't.
In that article, I compared the cost of a solar thermal system with the cost of an electric water heater and a PV system large enough to supply the water heater with enough electricity on an annual basis to heat enough gallons of hot water to satisfy the typical American family.
If you want to use PV instead of solar thermal collectors to heat your domestic hot water, there is no need to connect the PV modules to the water heater. As long as your customer lives in a grid-connected house, the operation of an electric water heater and a PV system are entirely independent.
The strength of a grid-connected PV system is that it simply feeds electricity into the grid when the sun is shining, running your meter backwards. The PV system is agnostic when it comes to how you choose to use your electricity. The entire purpose of the PV system is simply to lower your electric bill. The PV system doesn't care if you have an electric water heater or not.
So, go ahead and install an electric water heater (either a conventional electric-resistance water heater or a heat-pump water heater) if you want to heat your water with electricity.
And if you want to install a PV system, go ahead. But you don't have to. The two systems aren't connected.
For more information on heat-pump water heaters, see Heat-Pump Water Heaters Come of Age.
Martin, I hesitate to say that I think you may misundertand the idea behind the Sunbandit. As far as I understand it, the Sunbandit essentially uses the hot water as a "battery" in the true sens of the word. The tank loses 1/2 degree water temp per hour, so hot water created during the day could be available the next morning... This maybe alleviates some of Dana's continued concern of peak loads on a widely utilized net-metered solar community by moving some of the hot water creatiion loads completely off grid. Basically, it's creating a "partial off grid" situation. I, myself, feel like I am only 1/2 understanding the Sun Bandit, but something about it makes sense as compared to the decoupled resistance hot water heat + grid tied PV array.
Search ebay with "Solar-Water-Heater-EZ-DIY-Save-No-Pipe-Changes-Hot-Water-System-PV-MPPT" for a device that allows PV solar panels to be connected directly to an electric resistance water heater.
I'd be interesting in an analysis of the cost effectiveness (although when it comes to "green", that isn't the right metric).
Jon, I think the difference is that the Sun Bandit is wired and plumbed to allow for an alternate energy source to kick in if the sun energy runs out. That simple device you recommend might work for a preheater.
Apparently it leaves the upper element connected to the grid. But that may require a timer on the upper element.
Ethan,
Several of your comments over the last couple of days have been directed to Martin. He isn't ignoring you, he is away from GBA this week. I think he returns next Monday.
Ethan,
For more information on the Sun Bandit, see this article: PV Systems That Divert Surplus Power to a Water Heater.