The Basics of Solar Hot Water Systems

We humans use a lot of hot water in our modern lives – for washing clothes and dishes, cooking, cleaning our homes, bathing and showering, and even for recreational activities like saunas and swimming pools. Turning on the tap for access to warm water is something we all do, almost without thinking about it. But whether we realize it or not, our hot water habits add to our carbon footprints and our energy bills. In fact, according to the US Department of Energy (DOE), water heating can account for up to 25% of your home energy bill every month!

Believe it or not, installing a new solar water heating system (sometimes called a solar thermal system) in your home can decrease your water heating bills by 50% to 80% every year. It’s no surprise, then, that many consumers looking for ways to save on energy are turning to solar water heating as a way of reducing their environmental impact while shrinking their energy bills at the same time.

The Economics of a Solar Water Heating System

Of course, if you’re considering the purchase of a solar water heating system, the first thing you’ll want to evaluate is whether it makes financial sense. The good news is that a solar water heater system is one of the most cost-effective forms of renewable energy available to the average consumer today.

Determining solar water heating system size

The first step in determining the financial viability of a solar hot water system is to estimate your system size. There are several calculations you need to make in order to arrive at the final system size.

1. Evaluate solar resources: Solar water heaters work in almost any climate, even if you don’t receive a lot of sunshine, but have your solar system supplier do a site evaluation first. The US DOE has a list of software tools you can also try if you want to calculate the solar potential yourself.
2. Determine collector area required: Generally speaking, you’ll need 20 square feet (ft2) of collector for the first two family members, and then another 8 ft2 (if you live in the US Sun Belt) or 12-14 ft2 (in the northern US) for each additional family member. Be sure you have enough roof space for the collector size you want.
3. Estimate storage space requirements: Another space consideration is the amount of room you’ll need for a storage tank for your hot water (though this won’t likely be more than your conventional hot water tank). For a passive solar water heating system, factor in enough space for a tank that’s 50 to 60 gallons for up to three people, and 80 gallons for about four people. For an active solar water heater, calculate between 1.5 gallons and 2 gallons of storage for every ft2 of collector area.

The size and type of your system will largely determine how much money you’ll need to spend to purchase and install a solar hot water heater. That said, the average capital costs for a solar hot water system are between $1,800 and $4,500 depending on the size and system type chosen. Don’t forget to subtract any government tax incentives or rebates from the purchase price to determine the final capital cost for your system.

Estimating annual operating cost of your solar hot water system

In addition to the capital expenditure of your solar water heater, you’ll need to consider the costs associated with running it from year to year. The ideal would be for your solar hot water system to provide all of your water heating requirements, but for many people, having an auxiliary (back-up) gas or electric heating system is preferable. In addition to mostly low maintenance costs, running a back-up heating system is where you’ll incur some ongoing costs, which will be determined by the current energy rates offered by your local utility company.

 

For this calculation, you’ll first need to know the solar energy factor (SEF), which will fall between 1.0 and 11. This is a measure of the energy efficiency of the system since it shows the energy delivered by the system divided by the electrical or gas energy put into the system. As such, the higher the SEF, the more energy efficient the system is.

 

With this information along with your local gas or electricity price per therm or Btu, you can determine the annual cost of your system. These are the formulas you will need:

• For gas auxiliary system: 365 days × 0.4105/SEF × Fuel Cost (therm) = estimated annual cost of operation
• For electric auxiliary system: 365 days × 12.03/SEF × Electricity Cost (kWh)= estimated annual operating cost

So for example, with a gas energy cost of $0.95/therm and an SEF of 2.1, this will be the annual operating cost:

• 365 days × 0.4105/2.1 × $0.95/therm = $67.78 annually

 

To finally determine the average cost of investing and running a solar hot water system, divide the purchase price (minus any incentives or rebates) by 360 months (assuming you add the cost to a 30-year mortgage) and add the monthly operating cost (annual operating cost divided by 12 months).

For example, a system that costs $2,800 to install and has a $67.78 annual operating cost will cost approximately $13.43 per month to operate. Now compare that with the costs you normally incur to heat your water and determine whether the investment is worth it. The US DOE estimates that the average homeowner will pay only $13 to $20 per month for a new solar water heater when included in a 30-year mortgage – a savings that can add up to $500 every year. In the face of rising gas and electric costs nationwide, a solar water heater is normally a very sound investment.

Local bylaws that affect your solar water heater purchase

There’s one final consideration to make before deciding for sure that you will purchase a solar water heating system. You’ll want to check with your local community to find out if there are any local codes, covenants, or regulations that restrict the kinds of renewable energy systems you can install on your rooftop. If you install first and ask questions later, you may find yourself very disappointed if asked to remove it to adhere to local bylaws.

How Solar Hot Water Systems Work

Though a solar hot water system may have many mechanical components, it’s actually one of the most elegantly simple renewable energy systems you’ll find. The basic design of a solar hot water system involves running cold fluid through a series of tubes facing the sun on your rooftop to allow the solar energy to heat the liquid. The water is then brought back into your home for use in your laundry room, kitchen, bathroom, and pool or hot tub (the hot water may or may not be stored in a tank for later use).
There are, however, several different types of solar water heaters from which to choose. You’ll need to first decide the type of solar collectors you wish to employ, what heat transfer medium you’ll use (if any), the type of heat exchanger (if applicable), and whether you’ll use an active or passive system for circulating liquid through the system.

Types of solar hot water collectors

The solar collectors in a solar hot water system are the boxes that actually sit on your rooftop to collect the solar heat throughout the day. There are three main solar collectors used for solar water heaters in residential applications:

• Flat plate: These are weatherized boxes made with dark absorber plates of metal or plastic. These can be glazed (covered with clear plastic or glass) or unglazed depending on the local weather conditions.
• Evacuated tube: These are designed with parallel rows of layered glass tubes – the inner layer is a metal absorber with a fin and the outer tube is clear glass. The fin absorbs solar heat while preventing radiative heat loss.
• Integral-collector storage: Also called ICS or batch systems, these are designed with black tanks or tubes in a weatherized and glazed box to prevent heat loss. After cold fluid is preheated by running through the collector, it is sent to a storage tank in a conventional heating system which maintains the hot temperature.

Direct (open loop) vs. indirect (closed loop) heat transfer

The next consideration is whether you will heat your domestic water directly, or rely on a heat transfer medium instead.

 

Indirect or closed loop solar hot water heaters

 

An indirect or closed loop solar water heater involves circulating a heat transfer fluid or air through the system rather than the water used for domestic purposes. The heat transfer medium absorbs solar heat in the collectors, is circulated through the system, and then transfers the heat to the domestic water supply, which is usually stored in tanks. As such, these systems are slightly more complex than direct systems.

 

Normally the heat transfer fluid is a glycol-water mixture, though some systems use hydrocarbons (like an oil), refrigerants (such as that used in an air conditioning system), or silicones. Others are designed on an air-to-liquid heat exchanger method.

 

• Indirect advantages: These systems work well for areas with questionable water quality. And since the heat transfer fluid are less prone to freezing, a closed loop system is especially useful for regions where extremely cold temperatures are common.
• Indirect disadvantages: These do require more maintenance as the fluid needs to be checked every year and changed every 3-10 years.

 

One variation of the indirect heat transfer method is the drainback system. These use distilled water as the heat transfer fluid which is pumped into a separate “drainback” tank located indoors. This protects the distilled water from freezing temperatures and results in a more efficient transfer of heat to your water.

In an indirect solar water heater, there are several heat exchanger design options, including the following:

• Coil in tank: In this model, a coil of tubes rests inside the hot water storage tank, warming the water from the inside out. This is the least efficient heat exchanger model.
• Shell and tube: Unlike the coil-in-tank model, this one has an external heat exchanger made of two fluid loops installed inside a case. An internal row of tubes circulates the domestic water, and an outer layer of tubes circulates the heat transfer fluid.
• Tube in tube: In this highly efficient design, two tubes are nested together, one inside the other. The inner tube holds the domestic water and the outer tube holds the heat exchange fluid. The liquids flow in opposite directions.

Direct or open loop solar water heater

By contrast, direct or open loop solar hot water systems work by circulating the actual domestic water you’ll use through the collectors. This is a much simpler setup with both advantages and disadvantages.

• Direct advantages: Generally open loop systems are cheaper to install and require less maintenance over the lifespan of the system. And since the heat is transferred directly to the domestic water supply, it is more efficient.
• Direct disadvantages: Since water is more prone to freezing than the heat transfer fluids, this system is not suitable for areas that experience extensive or extreme cold weather.

Passive vs. active solar water heaters

Circulating water through your solar hot water system can either happen passively (without the use of a mechanical, energized pump) or actively (using an energized pumping system). A passive solar system relies on gravity and/or the natural buoyancy of warmer liquids to circulate fluids throughout the system. It’s a simple, cost-effective design.

A passive solar hot water system has the advantage of working in any climate even during a power outage. Without moving parts, passive solar water heaters are also very low-maintenance. Additionally, since it requires no energy to circulate the water, there are no additional operating costs for this aspect of the system. Their simple design also makes them ideal for DIYers interested in installing their own solar water heaters.

A very common version of the passive solar water heater is the thermosyphon (or thermosiphon) system. These are highly recognizable because they are designed with the tank located directly above the solar collectors (often on the roof). Operating on the principle that hot water rises, water is circulated naturally by convection. Cold water enters the bottom of the system and as it warms, it circulates to the top where it is stored in the tank. These systems require extra installation considerations given the added weight of water storage on the rooftop.

An active solar water heater, on the other hand, requires several extra components combined with power. These components include an electric pump, valves, and controllers for moving the liquid from inside the solar collectors into the home. As a result, an active solar water heating system can be more expensive than a passive system (both to install and to run) and they will not function in the event of a power outage. They can be, however, more efficient in transferring heat than passive solar designs and may be easier to retrofit as well.

Should You Purchase a Solar Water Heating System?

It’s true that purchasing and installing a solar hot water system for your home will require an up-front investment, but we’re confident that once you weigh the long-term benefits of this form of renewable energy that you will be convinced of the value in heating your water with solar. When you’re ready to make the purchase, talk to a qualified solar water heating professional who knows how to determine the best system for you and how to install it so that you can have hassle-free, low-cost hot water for many years to come.

And if you’re the type of person who wants to see solar water heaters become a reality for every home in America, consider embarking on a solar training journey to learn how to become a solar hot water installer or consultant. It’s a career move you won’t regret.