MAILBOX: What About Solar Water Heating?

We're all wet when it comes to water heating recommendations

I was looking at your off-grid solar water-heating article (“Off-Grid Water Heating" by Ian Woofenden & Vaughan Woodruff in HP177), and was disappointed by the lack of emphasis on renewable energy (except for solar thermal). In my house, we get most of our hot water from surpluses from the wind-electric and PV system, and that seems about right to me for a modern off-grid house. Often, we have more hot water than we know what to do with.

Solar and wind electricity were briefly mentioned, but the gist of your comment was that although possible, it was not a great idea. And it does not figure in the matrix of solutions at the end of the article.

I am a bit disappointed in Home Power lately with the emphasis on using propane. Propane fridge, propane hot water. If I were writing this article, I would not be telling people to choke their hot water taps so that they feel they are getting second-best service. I would be telling them to use abundant renewable energy. A lot of us longtime fans read it for the renewable stuff. Now, more than ever, we need more renewable energy, and it works better than ever!

As for solar thermal (although it is great), I think the money would be better spent oversizing an off-grid PV array. PV is really cheap. To provide ample electricity for as much of the year as possible, the system needs to be oversized. The result is loads of surplus energy in sunny weather that can be used to provide ample hot water. You will therefore not need solar thermal—that would be money wasted. Better to use the money to buy more PV, microhydro, or wind—whatever resource you have available at your site.

I agree with the two drawbacks you mention, but they are not serious. These surpluses are intermittent. Of course, you need a backup heating source—a wood heater or propane. It’s not hard to combine these with surplus electricity—it’s easier to do it with electric heat than solar thermal. Surpluses from a solar-electric system are inherently erratic, but if the system is properly designed, it can be a frequent and generous source of hot water.

You point out that it’s not possible to use hot water as your only diversion load. This can pose a technical challenge. You may need a combination of charge control equipment or relays to handle the surplus. But what I don’t understand is why there isn’t a hot water diversion output built into every large solar controller. The technology is simple. Using hot water as a priority and then reverting to a true “dump” is also simple to do, and it should be mainstream. Let’s ask the controller manufacturers to give us diversion with a solar priority and a backup space-heating output (if necessary) as standard in every controller.

This is the sort of off-grid water heating that we should aspire to, and it is achievable. I look to Home Power magazine to lead us toward this sort of solution rather than yesterday’s SWH and propane. Effective use of renewable energy is high on the agenda, both on and off the grid. Let’s get to grips with that.

Hugh Piggott • Scoraig, Scotland

Comments (3)

Joseph_Lull's picture

FLEXCHARGE makes such a controller - integrated diversion load control based off battery charge state.
I have used them, they are not long lived but are the step in the right direction.

Ryan Moy's picture

Hugh your response was dead on. Anyone reading this probably has a passion for using their own renewable energy but its great to see it so well articulated! You and Homepower are both right. Renewable energy needs to be priority but Homepower needs to prepare people for the realities of domestic hot water systems and until a brighter future day that means gas. Stinks, I know. I've been a "green" plumbing engineer for a while and stopped designing and specifying solar thermal systems. With PV supplied and heat pump water heaters available it just hasn't been the best system to install and i'm totally fine with that. Whatever gets me the most load off the utility grid is always my biggest goal and it's nice to have options.

Gerald Leap_2's picture

I agree completely with this. I have about 6000w of pv panels and a 35kwh battery bank. I use a shunt regulator connected to a 120gal 6000w electric water heater as a dump load; when the water reaches 180deg F the thermostat switches to a 6kw resistive load. A mixing valve provides 120deg water to taps. This keeps us in hot water for 5 days of cloudy weather. We have a propane backup that hasn't been needed in 5yrs. I am constantly amazed at how well this system works.

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