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When Does Solar Hot Water Make Sense?


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In this post I want to: make some initial comments on the recent study by Carl McDaniel and David Borton; share the results we got from installing our SDHW system; and introduce additional parameters and situations that should be considered. Next week I will run a few scenarios and discuss some of the other situations. I have also asked a couple local SDHW experts to comment and they may provide some data and experience. Michael Cellini of Allura provided a comment on the article last week, in case you missed it.

Let me start by saying, I never read an article or a book where I was so disbelieving of the general premise before I started. But, by the time I finished reading Carl McDaniel and David Borton’s article “Evacuated Tube Solar Hot Water Systems Are Not Energy Efficient or Cost Effective for Domestic Hot Water Heating in the Northeastern U.S. Climate,” I was agreeing. Every caveat I thought might provide an opening to rebut was meticulously documented and closed. The final touch was seeing the data after removing the solar system come so close to the expected results that I believe this to be an important article to understand. There are three main points:

  • First, using less hot water is the most cost effective way to save energy.
  • If you use very little hot water, for whatever reasons or by whatever methods, then investing in a solar domestic hot water (SDHW) system does not make sense.
  • Finally, we need to look at what a new system can save and compare that to the “best solution’s” costs and savings, rather than compare to what was installed previously.

First, my situation. We are fairly frugal with hot water. Wash clothes with cold almost all of the time; fairly short showers when needed; dishwasher runs only when full; etc. As a family of four we have used approximately 2500 kWh a year to heat water. I know this because I have a separate meter on my water heater. For four years before installing the SDHW we actually used 2428, 2479, 2519 and 2490 kWh. This is significantly less than what most of the literature says a family of four uses. The primary reason we used less than average is because of our hot water conservation habits; the second reason is because our children were taking few showers – more on this later.

I installed four used flat-plate collectors (about 20 years old), totaling 80 square feet, facing due south, at 48 degrees tilt, on a ground mount, 33 feet from the house. Also installed an 80 gallon pre-heat tank (8 Y.O.), circulator (20 Y.O.) and controls (20 Y.O.). Paying just $500 for the key components, it still cost me $3500: to build the ground-rack; for copper pipe, fittings and accessories; for high temperature insulation; and for help with the plumbing and installation. The preheat tank was then plumbed so water heated by the sun is sent to the existing water heater for it to finish off, if needed, to 120 degrees.

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Left Picture: Ditch 4' deep in heavy clay, treated ground mount frame.  Right Picture: Underground pipe insulated in 12" sono tube (back); Return pipe (behind collectors) insulated in 4" drainage PVC.

We turned the system on 9/3/2010. I then watched enthusiastically while over the next year we used a lot less electricity to heat the finishing tank to a constant 120 degrees – after a full year we had used just 1146 kWh, including an estimated 200 kWh for the circulator, which was not metered.

One complicating factor is our life changed on 9/7/2010 – the kids went back to school and Katie’s sixth grade class was told they should start following the western habit of showering daily! The nerve! Charlie, a bit compulsive and a keen observer, thought he should rid his body of 3rd grade “stink” and consequently more baths were taken! There may have also been a subconscious effort on my part to make the results look good by taking shorter showers? I can assure you no one else was so burdened. Consequently, I made what I feel is a conservative, upward adjustment to our “actual” energy needed to heat water without the sun to 3000 kWh.

If I had known we used 11,500 gallons before SDHW and 13,000 the first year, I could be much more certain in my adjustment. But we don’t know how much hot water we use. One of the inherent problems with “solving” an individual’s hot water problem is that the amount of hot water used is unknown. It is not unknowable, but a good estimating tool, application by an experienced contractor, and homeowner honesty are necessary to arrive at a good estimate, which as we will see later is important for a good decision.

Now, do the math: we saved 1854 kWh (3000 – 1146) @ $0.165 this equals $306 savings per year. Divide the $4,000 investment by $306 annual savings and I get a simple payback of about 13 years! A worst case evaluation would be a savings of 1,354 kWh (assuming only 2500 would have been required), $223 annual savings, or a payback of 18 years. Still not bad for a guy who does this stuff for reasons other than “just” to save money.

But what if… what if instead I had installed an on-demand electric water heater as Carl McDaniel did? The installation would have been a lot easier – one or two days instead of nearly two weeks! The resulting system would be simpler but with admittedly more technology. And it would have cost me less than $1000 to install, mostly by myself. I really don’t know how much hot water we take from the tap, but based on activities, frequencies, flow rates and rough run times, I estimated it is about 13,000 gallons annually for our family of four; and, yes there will be a water meter on the hot water in the new house. 13,000 gallons is about half of what “industry standards” estimate and this is consistent with our prior electric input for hot water. Note: Carl McDaniel and his wife use just 3000 gallons – this is extremely conservative and is probably not a reasonable goal for most families, especially those with kids! But, if we actually use 13,000 gallons of hot water a year, then raising the temperature 68 degrees using an on-demand water heater would require about 2228 kWh per year (13000 x 68 x 8.34) / (.97 x 3412); assuming manufacturer’s efficiency rating of 97% is correct. My savings would have been 772 kWh (3000 – 2228), or about $127 per year (771 x $0.165). My payback then would have been about 7.9 years ($1000/$127).

Clearly, we would have been better off financially installing an on-demand water heater! But, we would have used 1082 kWh more electricity (2228 – 1146) and thus less free sun! This looks like an economics question – at what point does the cost of electricity change the decision?

Still, there are other factors that should be examined relative to the performance and costs of my system before drawing conclusions about whether or not SDHW is right for you, for example:

  • Would I do better with an evacuated tube system than the flat plate collectors?
  • Would new flat plat collectors perform significantly better?
  • Did I pick the optimal collector tilt?
  • What if the distance between the collectors and the tank was less? More?
  • Would a roof mounted system perform better? (different tilt and orientation?)
  • Amount of insulation on the piping under the ground and through the basement?
  • Insulation on both tanks?
  • Are there more efficient circulator motors now?
  • Would more modern controls with greater flexibility produce better results?
  • What if our family used 20,000 gallons (say there were 6 kids)?
  • What is the net cost with incentives of a new, modern SDHW system installed?
  • Etc.

Plus there are other individual circumstances that should be considered, for example:

  • Is the installation for a house off the grid?
  • What alternative fuel is available for heating water?
  • What are expected fuel costs over time?
  • How available is the alternative fuel?
  • To what degree is waste heat from the tank appreciated in winter and not in summer?
  • System life expectations?
  • Amount and complexity of technology that is desired or acceptable?
  • The amount of expected maintenance?
  • The degree the homeowner wants to be 100% solar?
  • What else is the homeowner trying to achieve?
  • Risks of various weather and other disaster scenarios?
  • Etc.

Based on what I have seen of my system and what I have read of McDaniel’s results, I believe it is critically important to incorporate the above considerations and to evaluate any proposed system based on what could be and not on what has been. Generally, I expect we will find that households with very limited hot water demand will be better off with on-demand hot water system. But I also expect that households that use more hot water can save energy by first conserving hot water and then maybe by using the sun!

Next week, I will present additional information to help sort these issues out and perhaps we can get some local expert input as well.

Your comments and experiences are always appreciated!

Dan Gibson is the Reporter and Chief Coordinator of Our Energy Independence Community (www.OEIC.us). Previously he performed home energy audits for five years in NYSERDA’s Home Performance program and new home ratings in the New York ENERGY STAR Home program. He is currently building a 100% Solar Home. He can be reached at DanG@OEIC.us


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