We should probably all stop picking on solar thermal

Amy Musser

Is solar thermal dead?  Should it be?  Despite what you may read in popular blogs and hear from people around you, I don’t think so.  I have it in my own  house, I love it, and I feel like it’s time to lay out the case in its favor.

The key argument against using solar thermal to heat domestic  hot water is one of semi-simple economics.  The argument goes like this: heat pump water heaters and solar photovoltaic (PV) systems are now cheap enough that you can use these technologies together to generate carbon-neutral hot water more cost effectively than you can with a solar thermal system. The actual math is pretty straightforward, except that it relies on a web of tax credits and incentives that vary among various states and utilities.

In many cases, this may be true.  PV systems currently enjoy a 30% federal tax credit and a 35% tax credit in North Carolina (by the way, solar thermal also qualifies for both of these credits).  Customers of Progress Energy can participate in a great program called Sunsense to receive additional rebates.  Heat pump water heaters can be found on sale for about $1000 (plus install) and are about twice as efficient as the more traditional style electric water heater.  If you can stack up all of these rebates and incentives this very well may be the lowest-cost option.

Another major advantage of the PV plus heat pump water heater approach is that it’s more “plug and play” for the homeowner.  Solar thermal systems have a lot of moving parts.  They all come from different manufacturers and are put together by your installer.  Some of them will require maintenance or replacement.  You’re going to need to call a knowledgeable person to fix the system if it malfunctions.  On the other hand, PV panels tend to go on the roof and operate without much interaction from the homeowner for a long time (warranties are typically 20-30 years).  Heat pump water heaters aren’t going to last that long, but at least they are a single piece of equipment with a manufacturer’s warranty.

This all sounds great and for some homeowners this is absolutely the best way to go.  These are homeowners who pay enough taxes to make the credits worthwhile and who are connected to electric utilities that offer good payback rates for solar PV.  It includes homeowners who want a really low-maintenance system that takes up slightly less space and may not have seriously considered a solar thermal system to begin with.

So why should some of us still be considering solar thermal? 

One of the best arguments in favor of solar thermal isn’t mathematical.  I know it’s tempting to look at this as a simple math problem, but I have important news for you.  Decisions about your house aren’t as math-based as you think they are.  Not even when my house is involved and I’m an engineer.  I want them to be, but they’re just not.  Solar thermal is the only technology that will occasionally give you nearly-free hot water in abundance.  Why is this so cool?

soaking tub
Exhibit A – 80 gallon soaking tub

Which do you think would be more relaxing to think about on a Sunday afternoon? (a) you’re just drained your entire heat pump water heater which you had to size specifically to accommodate this tub, but you’ve installed enough PV to cover this indulgence OR (b) it’s a sunny day and your solar tank was 140 degrees, so the bath you are about to take is completely carbon and money free.

Exhibit B – white towels

Washing towels in hot water is really awesome.  Especially if you live in a humid climate where they spend the windows-open season perpetually in danger of getting funky, and especially if you like to use green laundry detergent.  Asheville has lots of sunny days all year round, and it’s pretty easy to save your “hot” load for a sunny day.

Don’t get me wrong, I like energy efficiency because I like using energy to do things that are useful to me.  But it’s not THAT hard to shift the use of your soaking tub and your hot-water laundry to a sunny day.  You look at the weather app on your phone and pick the weekend day that’s got the best weather, and it almost always works fabulously.

There are a few math-based reasons to give solar thermal serious consideration as well:

First, solar thermal panels will tolerate a wider variety of locations than PV panels.  PV panels are very sensitive to shading.  A small amount of shading (like the sun shining through the branches of a deciduous tree with no leaves in the winter) is more damaging to PV output than it is to solar thermal.  There are some strategies like micro-inverters that can make PV more tolerant, but solar thermal is still more friendly to these locations.

Second, the low cost of PV is still very dependent on tax credits and rebates.  These vary in every situation and remain under attack in places like North Carolina.  Not everyone pays enough taxes for credits to turn into money in their pocket.  If you’re retired, for instance, your taxable income and your tax bill  might be very low.  If you belong to one of North Carolina’s electricity co-ops, the buyback agreement for PV-generated power might not be great, and you won’t get a utility rebate.  This can change the math in favor of solar thermal pretty quickly.  Without incentives, solar thermal is still cost-competitive.  Likewise, if you want or need to be off-grid, solar thermal storage (a tank of water) is cheaper and easier to deal with than the equivalent capacity in batteries.

Third, not everyone has the up-front cash to make a big investment in renewable energy right away.  If you can only make a $5,000 – $7,000 up front investment, jumping into PV might not be your best choice.  You need to look carefully at how your utility buyback arrangement works.  The Progress Energy Sunsense program has a great payback for homeowners who can take simple steps to reduce their peak demand, but it’s difficult to add more panels later under that agreement.  The ideal way to participate in that program is to install a system that produces almost as much energy as you use throughout the year, without going over (they only pay you for as much power as you use in a calendar year).  If your budget says that you need to get started with renewable energy more incrementally, solar thermal might start to look more attractive as an initial investment.

Not everyone in America may be willing to make space for a solar thermal system and cultivate a relationship with a good service company to keep the system operating as intended.  But YOU might be.  And if you are, you might find that solar thermal has a lot to give back.

Copyright 2013 Amy Musser