Clearing up solar domestic hot water

Clearing up solar domestic hot water

Steve Shantz with a residential solar panel array.  Photo contributed by Steve Shantz.

Steve Shantz with a residential solar panel array. Photo contributed by Steve Shantz.

Letter to the editor

In the last year or so, the Record has published two articles about the proposed Recreation-Fitness Center solar hot water project. These articles have done a great job at presenting the concept, but have not been very descriptive of some of the issues involved.

The project started about a year and a half ago, when we learned that Indiana offers a matching grant of up to $25,000 for the installation of a solar domestic water heater (there have yet to be any takers for this money).

Domestic hot water is the stuff you shower in. The idea is to heat the water for the showers in the Recreation-Fitness Center. Three designs were submitted, and ultimately all of them were rejected for various reasons.

Basically, the designs were just scaled-up versions of a residential design. This doesn’t work for larger scale projects for the following reasons:

1.    The R.F.C. uses about 1,200 gallons of hot water per day. A good residential design should store two or three days worth of hot water in a tank to allow the system to deliver hot water for several cloudy days in a row. Unfortunately, storing that much pressurized hot water in a commercial scale tank costs about $10 per gallon. Do the math.

2.    A residential system typically uses antifreeze in the collectors and a heat exchanger to prevent winter freeze-up and destruction of the collectors and piping. This is very expensive.

For those of you who don’t know me well, one of my talents is designing and making things. I have designed and built two residential solar domestic hot water heating systems, as well as many other things in my previous job working in biochemistry, fermentation technology and medical diagnostics.

Designing and building things that work well is a passion for me. Not everything I have made has worked as expected, but each project has been a learning experience. Projects that didn’t work as planned are the very best teachers if one is willing to try to figure out what went wrong.

My initial disappointment on the death of the initial project led me to ponder what went wrong with the initial designs. The issues listed above are two key problems that need to be solved in order to make a viable system at a reasonable cost. Inexpensive storage of moderately low-grade solar heat is a difficult problem that people in the solar thermal industry wrestle with all the time. There are lots of ways to do it, but most are expensive, or complicated.

I spent a lot of time thinking how to do things better. Here is what I came up with:

1.    Heat from the solar collectors will be stored in an unpressurized, insulated cistern full of water, stored in the ground at the side of the building. This is relatively inexpensive to build, and allows the storage of four to five days worth of hot water. Also, we don’t have to worry about structural issues to support the weight of this much water.

2.    The collector system will be a “drain back” design. Cool water from the bottom of the cistern is pumped through the collectors, and the returned hot water naturally floats at the top of the cistern. No antifreeze or large heat exchanger is required.

3.    A modestly-sized pump will circulate the water up to the collectors and back down, utilizing the principle of a siphon to reduce electrical consumption.

4.    A small pressurized storage tank will be placed just before the existing boiler, which heats the hot water for the showers. The use of this “buffer” tank means that we don’t need a huge heat exchanger sized to meet the maximum demand for hot water.

Those are the basic design principles we are following. Fleshing out the design into a real system is the current challenge. It is amazing how many details need to be worked out. For example:

We had to verify that cold water and hot water will form a stable stratified system in the cistern and in the buffer tank. We modeled domestic hot water consumption patterns against different buffer tank sizes to determine an optimal size for the buffer tank.

How does one fasten the collectors to the roof of the pool so that the system doesn’t become an expensive kite in the first big wind storm?

The latest issue we have just become aware of is a pumping problem and the plan to use a siphon system to reduce electrical energy consumption. The current “siphon” design would result in water in the collectors at a pressure much lower than atmospheric pressure. We realized that as the water passes through the collectors and gets heated, it will likely boil at the reduced pressure in the collectors.

So there you have it. It is a design quite unlike anything else I have ever seen. But we believe it is a very good design. We hope to keep the system cost at $50,000 or less, but we are still working that out yet. Just for comparison, Governor’s State University on the south side of Chicago built a system almost three times as large as ours, at a cost of about $300,000.

The Goshen College administration has an amazing policy towards energy savings, with energy saving projects with paybacks of 5 – 6 years regularly getting approval. I want you students to know this! This policy is partly responsible for the dramatic drop in college electrical consumption in the last 3 – 4 years. However, the payback on the Solar DHW project is not so attractive. We will save at best, only $1500 – $2500 per year at current gas prices.

Some questions that I have pondered:

If the college is willing to put up as much as $25,000 for such a system, is this the best use of that money?

Could this money be put to better use on other energy saving projects with better paybacks? This question is a difficult one for me to ask.

How is the decision to go ahead with such a project affected by non-economic factors such as educational value, a reduced carbon footprint or the attractiveness of the idea to prospective students and the community?

Could this project somehow help motivate the community to consider energy saving projects that might not be attractive based on a simple calculation of payback?

Finally, what might come next? Solar domestic hot water for the dorms and the apartments?

Hey, I can dream right! Watch out. We dreamers are a dangerous lot.

Steve Shantz is a systems operation technician at Goshen College.

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