Solar Thermal is Dead (NOT!)


I should start out by saying that I like a lot of the articles that Martin Holladay puts up on the site.  They often bring out new points of view and promote some good thought and discussion on topics that don’t get addressed a lot elsewhere.

Martin recently wrote a blog entitled “Solar Thermal Is Dead”, in which he argues for the idea that for heating domestic water it is more cost effective to install a combination of a solar PV system plus a heat pump water heater than it is to install a solar thermal water heating system. 

I think that he got some things wrong in this article, and since I’ve been getting questions on it, I decided to layout why I disagree with his assertion.

I want to say up front that I am a fan of solar thermal – I’m an old airplane engineer and the simplicity and efficiency of these systems is very appealing to me.  So, as much as Martin heavily weighed the dice in favor of PV + HPWH, I may have weighed the dice a bit in the other direction.  You can read through and make up your own mind.


The material below goes through the article, and than makes a few points not covered by the article at the end. It then covers the situation for DIY builders, which is much different.

The Main Premise of the Article:

Martin quotes from a conversation with Charles Stephens to get across the main premise of the article:

“If you want to do solar water heating and solar space heating, solar thermal remains too expensive,” Stephens told me. “It’s not as cost-effective as using an air-source heat pump coupled to a PV array. In our climate, a properly sized solar thermal system can provide 100 percent of your hot water in the summertime, but it won’t do diddly in the wintertime. So you paid $4,000 for a system that provides 40 or 50 percent of your hot water needs. If instead, using the same money, you just add an extra kilowatt of PV to the roof, you could heat all of your hot water year round with an air-source heat pump.”

The 40 to 50% solar fraction seems very low to me, even for a tough solar climate like Oregon.  The RETScreen tool says that a solar water heating system that is designed to deliver 48 gallons of hot water per day with 65 sf of collector would achieve a 72% solar fraction in Portland, OR.  In Denver, the same system would deliver a 92% solar fraction.

RETScreen is a widely recognized tool that is used (and even required) for analysis of solar heating system by many agencies.

So, this seems like a case of 1) picking a very non-typical and poor solar location to do the comparison, and 2) using a strangely low ball number for the solar fraction even for that poor location.

Of course, the same poor winter solar climate in western Oregon would, of course, also reduce the output of the PV system in the winter.

Martin article says: Some Solar-Heated Water Goes to Waste
Martin makes the point that solar thermal systems produce more heat in the summer than a family can use.  This is likely true, but does not seem like a disadvantage to me.  What’s wrong with a system that satisfies all of your demand with some margin?   It really only makes a difference if the cost of such a system is more than the cost of a PV + heat pump system that meets the same demand. 

Of course, a PV system has the same characteristics of producing more than the demand in the summer and less than the demand in the winter.  The difference is that a grid-tied system coupled with most of the current net metering agreements can effectively use the grid as seasonal storage.  You sell the excess power to the grid in the summer and use grid power to make up for short falls in the winter.  The net metering agreements as they are now in which the utility normally pays full retail value for the homeowner PV production is very generous to owners (like me) of PV systems, but this is unlikely to continue.  This idea of paying PV producers full retail for the electricity they produce clearly is not workable when the fraction of power produced by PV increases to significant level.  Many states and utilities have realized this and are reviewing and changing net metering agreements in some way.  I think that this trend will only increase over time.  The net effect of this will be to tip the economic advantage toward solar thermal systems.

Martin Article Section: Comparing solar thermal and PV systems

"Compared to a PV system, a solar thermal system has several disadvantages:

I think this is way off the mark for these reasons...

Using the drain back solar water heating system design as an example -- it is about as simple as it gets.  It has one moving part (the pump).  These pumps are usually the very tough and reliable Taco or Grundfos HVAC pumps – they have a deserved reputation for long life and reliable operation.  The collectors are very long lived and have a solid track record of 30+ year life.  The only other part in a drain back system that is subject to failure is the differential controller and its sensors.  These seem to have poorer reliability than one would expect from such a simple control, but should the controller fail, there are good quality replacement controllers for as little as $100.  The drain back systems require no regular maintenance.

There are more complex forms of solar water heating systems that may require more attention, but all of them are far far far simpler than a PV system PLUS a heat pump water heater.

In contrast, A PV system has PV modules which are subject to failure (causing reduced output that is often not detected), a very complex inverter, switches, circuit breakers, combiner boxes, and wiring.  Add to this the heat pump water heater which has a pump (sometimes the same kind of pump used on a solar water heating system), a compressor and compressor motor, a fan, heat pump controls, a high pressure refrigerant circulation system.  This has to be several times the complexity of a simple solar thermal water heating system.  I simply cannot believe that such a system will be longer lived and more trouble free than a solar thermal water heating system.

In addition, heat pump water heaters have other not so good characteristics: 1) in the winter (depending on where in the house the HPWH is installed) the effective COP can be as low as 1 because the heat that the HPWH adds to the water is taken from the house air and just has to be reheated by the furnace – HPWHs can be just as inefficient as a regular electric resistance water heater in the winter.  2) all you have to do is a little Googling on reliability of HPWHs or read some of the reviews on Amazon to show that at least some HPWH’s have poor performance, low reliability, and short lives.  3) HPWHs can be annoyingly noisy.  I’m not saying that HPWHs are not a good solution in some cases or that they don’t work well for some people, but to say that they are some kind of paragon of reliability, performance and long life seems like a very big stretch to me.

A solar thermal water heating system should last 30 years with minimal maintenance and repair.  Generally the parts that fail on a solar thermal water heating system are relatively inexpensive.  Contrast that with a PV system in which the thousand plus dollar inverter has an expected life of perhaps 10 to 15 years?  And, the expensive HPWH having an expected life of 10 to 12 years?

Martin article says: Let's do the math

In this section Martin compares a solar thermal system costing $10,000 to a 1.7 KWH PV system at $7,000 plus a HPWP and $3,000.  He says the $10K solar thermal system produces a solar fraction of 63%.  He says that the PV + HPWH provides 100% of the water heating demand.  So, his basic assertion is that both systems cost about the same, and the PV + HPWH has a higher solar fraction, so why not choose it?

His cost for the PV system and HPWH and the PV energy produced per year all seem reasonable to me.

The cost of the solar thermal system seems very high to me.  Here is Bozeman where I am the local solar installer does high quality system for $6000.  I suppose this varies from place to place to place, but $10K seems very high to me.  Simple enough to check the prices for both systems where you are.

In estimating the gallons of water heated by the PV + HPWH, Martin does not include the fact that the effective COP of the HPWH in the winter may be well below 2.0 if the HPWH is installed anywhere in a conditioned or semi conditioned area of the house. This is because the furnace will have to reheat the house air that the HPWH cools.  If the HPWH is within the conditioned space, then the effective COP for the full heating season will be more like 1.0 rather than 2.0. If you include the inefficiency of the furnace, I guess the effective COP could drop below 1.0 in the winter -- that is, if the HPWH takes 10K BTU out of the house air, an 80% efficient furnace will use 12.5K BTU of fuel to add the 10K BTU back into the house air.

If you assume a COP of 1.0 for 5 months and 2.0 for 7 months, then the yearly average COP falls from 2.0 for the year to 1.58 and the number of PV KWH that go into water heating falls from 4000 KWH down to 3160 KWH.

The 63% solar fraction that Martin uses for the solar thermal system in Boston seems low to me.
Using RETScreen, the predicted solar fraction for a 65 sqft collector area in Boston producing 48 gallons of hot water a day is 84%.  In Denver it is 92%.  Even in Seattle it is 65%.

I use 48 gallons demand for a typical family rather than 64 gallons Martin starts with because I believe it is closer to actual typical family demand.

So, if you redo Martin's comparison considering the paragraphs above, it comes out like this:
The $6000 to $7000 (not $10,000) solar thermal system provides 84% (not 63%) of the demand of a typical family in Boston, and it does this without any dependence on the current generous net metering rules. 

The $10000 PV plus HPWH system with an average yearly effective COP of 1.58 (not 2.0) produces 3160 KWH a year (not 4000).  This is enough to heat an average of 50 gallons of water from 50F to 120F per day. So, for a demand of 48 gallons per day, the PV + HPWH has a solar fraction of 100%.

So, comparing the two systems:
The solar thermal system costs $6 to $7K and produces a solar fraction of 84% in Boston.

The PV + HPWH systems costs $10K and produces a solar fraction of 100% in Boston.

In this revised comparison, the PV + HPWH produces a 19% higher solar fraction, but its initial cost is 54% higher than the solar thermal system. This seems like a net win for the solar thermal to me.

Again, the performance of the PV + HPWH is totally dependent on the full retail net metering, which is unlikely to continue in its current form. If the PV + HPWH was in an off grid situation and was required to provide 48 gallons of heated water a day, and it operates with a mid winter COP of 1.0 in Boston, an about 3.5 KWH PV array would be required. This would be much more expensive, and take up 250 sqft of roof space.

As to maintenance over the years, during the 30 year life of the solar thermal system the PV + HPWH will likely go through 1 or 2 thousand dollar plus inverters and 2 or 3 thousand dollar plus HPWHs.  The solar thermal system will likely require a new pump at some point for a couple hundred dollars and a differential controller at some point for a couple hundred dollars.  During this entire time, the PV panels will be losing a little of their output each year, and some of them may suffer sealing failures or cell connection failures – things that are difficult to detect, but that degrade performance.

Some items not covered in Martin's article

Some basic physics of the systems:

It’s important to consider some basic differences in these systems. 

A PV system with usual PVWatts losses considered has a collection efficiency of about 12%.

A solar thermal system has a collection efficiency of the order of 50% depending on ambient temperatures and sun levels. 

So, the thermal collectors are 4.2 a times more efficient. This is a huge difference.  If batteries could improve by this factor we would all be driving electric cars.

If you were to compare a system in which the HPWH was not scabbed on to improve the overall system efficiency, then you would need about 270 sqft of PV modules to have the same water heating output of the 65 sqft of collector needed for the solar thermal system.

Even the HPWH with a COP of 2.0, the effective efficiency of the PV + HPWH system is only about 24% -- half that of the solar thermal system.  And, to get this you have had to add on a complex, not so long lived, maintenance prone HPWH.

The only thing that makes the PV + HPWH even remotely competitive with the solar thermal system is the current very generous net metering rules, and this (I think) is shaky ground to stand on.

I do agree with Martin on these points:

In addition for northern climates where the bulk of energy use is space heating, it seems to me that it makes sense to do a combined space and water heating system – shared components and a combined install can lower the price and complexity of the combined system.

As an aside, I also think that we tend to miss a fundamental truth about heating water for domestic use.  It’s a really wasteful process.  We take a shower and whatever heats the water for it spends a ton of energy heating the water from 50F up to 120F.  This water flows over us once and goes down the drain taking 95% of the energy we just put into it right down the drain.  Maybe the real answer is a simple and low cost heat recovery design for the drain water built into every new house?

The DIY Picture

Build-It-Solar is 90% a DIY site for people who want to build their own systems.  All of the discussion above applies only to commercially installed systems.
In the commercially installed systems world, the two approaches end up in the same ball park, but for the DIY case, they are not even close.

The DIYer can build a solar thermal system equivalent to the $6000 to $10,000 commercial system described above for not much more than $1000.  This is not an exaggeration – my initial domestic water heating system cost $1000 and I have heard from many people who have built similar systems from $500 to $2000.  A number of these systems are described in detailed here...

The payback period for these system is typically only about 3 years depending on cost, climate, and fuel used on the old system. 
Of course, the same DIY approach can be taken for the PV + HPWH system, but the savings for doing it yourself are not nearly as large because the off the shelf components are expensive.

April 23, 2014



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