Solar Thermal Hot Water - A Cost-Effective Solution

The first thing that most people think of when the topic of "solar energy" systems is raised are "solar panels".  By this they think of making electricity from sunlight through the use of photo-voltaic (PV) solar panels.  Certainly, this is a hot topic with enormous potential today and even greater potential in the future with major advances being made every couple of months!

Less well-known, however, and with much less of a "wow!" factor among the general public, are solar thermal systems.  Solar thermal systems capture heat from the sun and use it in some way.  While there are an increasing number of commercial facilities that use this heat to generate electricity by heating fluid and using it to generate turbines, most solar thermal systems are much more low-tech (and thus, much more affordable for small-scale use by homeowners and small businesses) and use that heat to heat water.  We call these systems solar thermal hot water.

On this page you may find some answers to a few common questions regarding solar thermal hot water systems. EnergyActionNH can help you learn more about this and other renewable energy technologies and can help you evaluate whether it is right for your home or business.  If you should decide you want to install a system, we can help you find an experienced local contractor or, for those homeowners who would like to keep installation costs to a minimum and are willing to learn by helping others install their systems, there is the option of the community, pay-if-forward, barn-raising style Energy Raiser. Click here for more details.

Is Solar Thermal Hot Water New?

Solar thermal hot water systems have been around for decades.  In fact, there are several homes here in the Capitol Region of New Hampshire that have had solar thermal hot water since the 1970's.  While a few of the components of today's systems may be a bit more high-tech, for the most part, the technology and the process is the same today as thirty years ago.

Can I Heat My Home With Solar Thermal Hot Water?

While you can use solar thermal hot water with under-floor radiant heat systems, these systems need to be larger so they can capture more heat during the winter, when you need to heat your home, but also have a place to use that additional heat in the other seasons, such as to heat a swimming pool.  There are several examples of these types of larger systems in central New Hampshire, but they are less common than systems that simply use the heat to help heat water for domestic uses like bathing, washing clothes and dishes, etc.

What if it's Cloudy? Will I have to take Cold Showers??

Unlike photo-voltaic systems, solar thermal systems may generate some energy on lightly cloudy days.  However, solar thermal hot water systems are almost always built with a backup energy source.  Some systems are designed to boost incoming water to pre-heat it to minimize the BTUs required for the primary fuel source to generate.  Other systems are built so that the solar thermal collector is providing most or all of the heat required during the spring, summer and autumn, relying on the backup source on cloudy days and later in the autumn, the winter and early spring when there is less sun.  For these systems, people often use a solar thermal hot water tank that has two heating coils -- one for the solar thermal loop from the solar collector, and the secondary an electric or gas/oil heated water coil coming from an existing furnace or boiler. 

What does a Solar Thermal Hot Water System Cost?

The cost of a system can vary widely, depending upon a number of factors including the size and type of the collector, whether you are planning on using an existing tank and other components and whether you are planning on hiring contractors to all of the work or are planning on doing some or all of the work yourself.  A system could cost as little as $2500 and go up to $8000 or more. 

What is the Payback Period for Solar Thermal Hot Water?

Again, there are a lot of variables that affect this. Solar thermal hot water systems, however, are currently considered to have the shortest payback period among the most common small renewable energy systems found at homes and small business (i.e. solar thermal hot water, PV and wind generators).  With the current federal incentives and the high cost of electricity in New Hampshire (currently around $0.16 per kWh), the payback period of a typical $6000 system is well under ten years.  As energy prices go up, your payback period gets shorter and shorter.  Once you reach the end of that theoretical payback period, you are getting nearly free BTUs from your system.  It's really a mindset.  If you choose to only think of things in terms of "what's the cheapest thing for me right now", you'll never install one because by nature, you must invest something now in order to save later.  Sort of like your retirement account, in a way!

What if I Go Away for a Few Days?

Under normal circumstances, you are using hot water on a daily basis, drawing hot water out of your tank and, thus, drawing cold water into the bottom of your tank.  This keeps bringing down the temperature of your tank and giving your solar collector something to heat.

When you go away for a few days and stop using hot water, if you have a system designed to provide most of your hot water, the possibility exists that the temperature in your tank could keep building if there are several sunny days in a row.

There are several things you can do to ensure that your system never overheats in this situation.  One solution is to simply cover your collector so it is not collecting the heat from the sunlight.  If your system is on the roof, that is not practical. 

We recommend that you install a system with a temperature differential controller that has a vacation mode setting.  Normally, your temperature differential controller measures the temperature at the collector and in the tank and, when the collector temperature is more than about 14 degrees than the temperature in the tank, it turns on your circulator pump to circulate the glycol mixture through the solar loop, bringing heat from the collector to the tank.

Vacation mode, however, will detect when your collector is cooler than your tank, usually at night, and turn on your circulator pump to dump heat from your tank to your collector to help cool your tank.

Another thing that you can do to anticipate this issue when you install your system is to install a heat dissipator with a valve that you can turn when you are going away to divert the glycol mixture through the heat dissipator on its way to the tank to dissipate some of the heat before it reaches the tank.  You can purchase a heat dissipator specially made for the purpose or simply use a section from an old hydronic (hot water baseboard) heating system.

What Kind of Maintenance is Required?

Under normal circumstances, virtually no maintenance is required for solar thermal hot water systems on a day-to-day basis.  Every few years you may wish to check the glycol/water mixture in your solar loop, as you would check the glycol/water solution in your car radiator.  This is very easy to do.  The glycol mixture may need replacing at some point but it should last several years and is easy and fairly inexpensive to replace.

Renewable Energy Systems as "Pre-Paid Energy"

Although more and more people are installing larger renewable energy systems in their homes and businesses, most people should think of small-scale renewable energy systems of all kinds, solar thermal, PV and wind, more in terms of supplemental, or "pre-paid energy" systems, rather than thinking that they will replace all of the existing sources of energy in their homes and businesses.  The actual long-term cost per BTU or kilowatt hour is impossible to know for certain because we don't know what future energy prices will be.

However, given our rapidly growing world population, more competition for known energy reserves and rapidly rising extraction costs due to the fact the "easy-to-extract" fuels are largely gone, prices are going to go up, higher and higher, faster and faster.  We just don't know how high and how fast.

So, are you a person who thinks energy prices aren't going to change all that much or do you think that locking in the price of most of your hot water for the next two or three decades (or more!) is a good investment?

If you are in the latter category and want to know more, read more about our organization, about Energy Raisers and then come to one of our monthly Energy Exchange meetings to find out more!