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The Fundamentals and Economics of Solar Hot Water

by Tom Hughes

Have you ever climbed into a car on a sunny day and found the seat uncomfortably warm, the steering wheel too hot to hold, and your candy bar on the dashboard a melted glob of goo?  Congratulations!  You have just experienced the fundamental concept behind every solar hot water system – to capture the sun’s radiation and convert it into heat.

That captured energy is then used to heat the water used in your home.

In many northeast households, the cost of heating the water for showers, laundry and washing dishes amounts to as much as 30% of the overall household heating bill. That can be hundreds of dollars a year washed down the drain.

A solar hot water system is a great way to reduce that expense.

By using the sun to pre-heat the water that flows into your existing hot water tank, your existing hot water tank won’t have to work so hard. And you’ll save money on your monthly heating bill.

Even better – with today’s low cost systems, federal tax credits, state incentives and attractive financing opportunities – you can “break even” on your system in Year One. If the rate of inflation remains the same a family of four could save $20,000 on their fuel bills in the first twenty years of ownership.

System Basics

Almost all solar hot water systems (also known as solar thermal systems) in New England have five key components: solar collectors, heat transfer fluid, a pump, a heat exchanger and a storage tank.

The collectors can be mounted on the roof of your house or on a frame in the yard. Their job is to collect the sun’s energy. The more sunlight they receive, the better they will perform. Generally, you’ll want to have your collectors pointed as close to due south as possible and tilted at an angle equal to your latitude. If you’re a few degrees off left or right or up or down, that’s OK.

When the sun shines on the collector, the black metal plates under the glass absorb the sun’s radiation and get hot – just like the inside of your car on a sunny day.

Connected to the black metal plates are tubes, generally made of copper, with a heat transfer fluid flowing inside.  (Here in the northeast, we need something hardy that can withstand freezing temperatures so most systems use a glycol solution as the heat transfer fluid. Glycol doesn’t freeze at the same temperature as water and that protects the tubes from bursting in the winter.)

As the sun warms the metal plates inside the collectors, they in turn heat the fluid inside the pipes. That hot fluid then needs to get to the heat exchanger, which is usually in the utility room inside the home. This is where the pump comes into play.

The pump circulates the heat transfer fluid through a loop of tubing from the collectors to the heat exchanger.

The heat exchanger is where the magic happens. Inside the heat exchanger, the free energy from the sun is transformed into useful energy for your home. How big is this magical device? (All this happens in a little box next to your water heater….)

Hot fluid is pumped from the collectors to the heat exchanger. Inside the heat exchanger, cold water coming into the home circulates around the hot copper tubes, drawing the heat off the pipes.

That’s it!  That’s how sun heats your hot water and saves you money.

The now hot water flows into the storage tank for future use. The now cool glycol fluid gets pumped back to the solar collectors to be re-heated and circulate through the loop again.

The storage tank is placed near your existing water tank. Any time you turn on a hot water tap your existing hot water system will draw in pre-heated solar water –instead of cold well or city water – and will do less work.

A solar thermal system can pre-heat water for any type of existing water heater: fuel oil, propane, electricity, natural gas, wood or pellets. They can work with traditional storage systems or on-demand units. Regardless of how you heat your household water now, a solar hot water system can reduce your monthly heating bills and greenhouse gas emissions.

Economic Basics

The economics of a solar hot water system are simple – you’ll spend less on your monthly heating bills and the up-front expenses of the system are reduced by evolutions in system design that have reduced costs, tax credits and state and local incentives.

While the fundamentals of solar hot water system design have not changed dramatically in the last few decades, the industry has taken advantage of the surge of innovations elsewhere in renewable energy installation. Household systems can now be installed in as little as a few hours, saving hundreds or thousands of dollars in labor costs. A typical residential solar hot water system costs between $7500 and $10,000 after all of the expenses.

The total expense is eligible for a 30% federal tax credit. State incentives can reduce the costs by another 10-15% or more. Together, those savings can bring the costs of ownership down by 50% for many families, and bring the typical system down to $5000 after all the expenses.

Take advantage of the historic low rates on financing these days, and it is possible to be “cash positive” for every year you own the system. That means the payback period for your system can be less than one year!

With the high cost of energy, the uncertain future of inflation rates, the low cost of financing and the generous credits and incentives available, there has never been a better time to install a solar hot water system.

If you do, the next time you get into a hot car you’ll smile to yourself knowing that you’re saving money at home.

Green Energy Times wants to thank Sunward Systems, LLC from Vergennes, VT for this excellent Solar Hot Water education that should help to create the needed understanding to you all“.

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