In my life B.C. (Before Children), I received my Masters Degree in Mechanical Engineering with an emphasis is renewable energy. After graduation, I worked for a company that operated the world’s largest flat plate collector solar water heating (SWH) system and did SWH installations. When we built our current home, we added a solar water system shortly after the home was completed. It’s been providing up with hot water since 2006. (We also have passive solar heating in our current home.) Basically, I’ve had solar water heating in my life for over 20 years now, living in an area where you might least expect to see it (Wisconsin).
In this post, I’ll give you an overview of solar water heating basics so you can decide if this green home option is right for you.
Why use solar water heating?
After heating and cooling, water heating tends to be one of areas where people use the most energy. Estimates range from around 15% to up to 40% of energy usage, with the high end coming from old, inefficient electric heaters.
With solar water heating, you use heat from the sun to heat some or all of your water. At its most basic, this can be done with a dark container left out in the sun. Friends of ours camped out on their property while building their home, and set up an outdoor shower with 50 gallon barrels painted black on a platform above their shower area. Another variant of this is the solar camp shower bags that can be filled and hung in a sunny location to provide hot water while camping (or during an emergency situation).
In our case, our solar collectors preheat water for our domestic hot water and for our in floor radiant heating. On sunny winter days, the passive and active solar cover the bulk of our heating load.
Pool heating is a great application for solar water heating, because pool usage is heaviest when the sun is most abundant.
Indirect Versus Direct Solar Water Heaters
Solar water heating systems can be classified in several ways. One of the main classifications is Direct or Indirect.
Direct systems heat the water that is being used. There is no heat exchanger. These systems work best in warm weather areas, because they must be drained when temperatures get below freezing. (Direct systems that are drained during cold weather are called, not too surprisingly, Drain Back Systems.) These systems are also prone to hard water buildup if the water running through them is hard. These are also known as open loop systems.
Indirect systems heat a fluid (generally an antifreeze mix) and transfer heat from that fluid through a heat exchanger to the potable water in the storage tank. These are a must for year round operation in cold climates. Because you don’t introduce new fluid into the collector, they are less likely to get mineral buildup. The down side is that these will generally be more expensive because they have more parts. These are also known as closed loop systems.
Passive versus Active Solar Water Heaters
Passive solar water heaters rely on natural convection to move the cold water from the bottom of the collector to the top as it is heated.
Active solar water heating systems have a pump to move the fluid around.
Parts of a Solar Water Heating System
Collector – All SWH systems need a way to collect the heat. Options range from a black painted tank or a black tank in an insulated box (batch heaters), to flat plate collectors and insulated tube collectors.
- Batch heaters combine the collector and storage in one unit.
- Flat plate collectors have fluid piping connected in parallel (typically copper tubing) with metal fins painted black to increase the absorption area, all inclosed in an insulated box.
- Insulated tube collectors are individual tubes with fluid pathways and fins inclosed in what is basically a thermos. No air around the absorbers = no air flow = superior insulation. If we were to expand our system, I’d want these collectors, since they have the lowest amount of heat loss in cold climates.
Storage tank – Where the heated water is stored.
- Batch heaters store the water right in the panel, tank, or storage tank attached to the top of the panel
- Flat plate collectors and insulated tube collectors have a separate storage tank.
Depending on the type of system, it may or may not include the following parts.
Heat Exchanger – In indirect systems, the heat from the solar panels is transferred to the domestic hot water via a heat exchanger. This heat exchanger may be located inside the storage tank or outside the storage tank. Inside the storage tank generally improves heat transfer, because it maximizes the amount of water surface area in contact with the heat exchanger, but it there is ever a problem with the heat exchanger, the entire tank and heat exchanger combination needs to be replaced. With an external heat exchanger, they can often be plumbed into a standard water electric water heating tank with the heating elements removed, which reduces costs, but they may not be as efficient as an internal exchanger. Our current system has an internal heat exchanger, our last system had an external heat exchanger.
Controller – In indirect systems with an AC pump, there is a controller to tell the pump when to turn off and on. This is also commonly referred to as a differential controller, because it looks at the temperature differential between the panels and the water in the storage tank. When a set temperature difference is reached (when the panels get hot enough), the pump turns on. When the panels cool down, the pump turns off.
Pump – Indirect systems may have an AC pump or a DC pump. AC pumps are typically powered by the electrical grid. DC pumps are almost always powered by a solar electric (photovoltaic) panel. As mentioned above, AC pumps are turned off and on by a controller. DC pumps turn on automatically when the solar electric panel generates enough power to turn on the pump. When sized correctly, the fluid in the panels should be hot enough to effectively transfer heat when the pump has enough power to operate. Our previous system used a DC pump and solar electric panel. Our current system uses an AC pump with controller. Our solar installer indicated that he had seen significantly better performance in our cold temperatures with the AC pumps, because the DC pumps didn’t have enough power to move the fluid well when it got cold and slushy.
Temperature and Pressure Gauge – Most system with inside storage will have one of these gauges near the storage tank so you can easily see at a glace the current conditions of the system, and make sure that it is properly pressurized.
Expansion Tank – Water, even water with antifreeze mixed in, will expand and contract as it goes through different temperature and pressure ranges. To give the excess volume of fluid somewhere to go in closed loop systems, a tank with a bladder that expands and contracts is plumbed into the loop.
Temperature and pressure relief valve – as extra protection against overheating, many systems place a temperature and pressure relief valve at the top of the collector set up. If the panels are left sitting in the sun with no load and get dangerously hot, fluid will be vented outside the house to avoid damage inside.
Things to Look for in a Solar Water Heating Installation
Make sure your solar system is installed correctly
Back in the 1970’s when there were a ton of rebates available, there were many contractors who installed sub par equipment, and did it poorly. One of the first things I did at the solar contractor I worked for was to go out and repair non-functioning systems as part of the Orphan Solar Program. You can’t take short cuts or the system either won’t work right from the start or will have a very short lifespan. When these puppies leak, they can make a really big mess, so it’s not generally a project for an unskilled homeowner (unless you’re doing something very simple like seasonal pool heating). Also, improperly mounted panels on the roof can cause major roof damage and even structural failure. Water is heavy, and roof penetrations should always be properly sealed. Whether mounted on the ground or roof, you want to make sure your panels are very secure, so they don’t act like sails when a good windstorm kicks up.
Work with quality equipment
The SRCC (Solar Rating and Certification Corporation) tests panels and systems to verify that they deliver the energy they promise. If your equipment isn’t SRCC rated, at least make sure it comes with a warranty. Solar water heating equipment can endure some very extreme temperature and pressure ranges, and not all material will stand up to the punishment. I’ve heard about more than one situation where a homeowner thought they could save money by running standard PEX tubing from the collectors to the house, only to have the tubing rupture under normal (for solar) operating conditions.
Look for an installer with references
If you hire an installer, make sure they know what they’re doing. The North American Board of Certified Energy Practitioners (NABCEP) offers entry level knowledge assessment, professional certification, and company accreditation programs to renewable energy professionals throughout North America. There are also a variety of local training programs in some states. Your system will be a significant investment, so make sure you work with someone you trust.
Take advantage rebates and incentives to reduce your system cost
DSIRE is the Database of State Incentives for Renewables and Efficiency. Depending on your location, these incentives may help you to dramatically reduce the cost of your solar water heating system, so it pays to take a look.
Given that this article is already getting quite long, I’m going wrap things up here. Hopefully this has given you a basic overview of how solar water heating works.
You may also enjoy:
- Solar Electric Basics and our other posts on solar electricity
- The Building an Eco-Home series
- Living in a Concrete Bunker – part one and part two