Solar energy is making waves as the popularity of PV panels skyrocketed over the past decade. But using the power of the sun isn’t limited to photovoltaics. The raw heat energy of the sun can also be used to heat your domestic water supply. Traditional water heaters comprise a large chunk of residential energy usage, so installing a solar water heating system can save you hundreds, if not thousands, of dollars over the course of a year. Continue reading to get the low-down on how solar water heating systems can save you money!
Solar water heaters rely on the sun to heat your water. The water is run through solar collectors, where the heat is absorbed before being stored in an insulated storage tank. They can be used for hot showers, pools, or even space heating. Solar water heaters come in several different designs. Finding the best type of solar heater for your needs depends on various factors, including local climate, daily water use, and budget constraints.
How do Solar Water Heaters Work?
Solar water heaters provide hot water for domestic use by harnessing the power of the sun. They run water through a solar collector mounted on your roof, and the hot water is then kept in a storage tank for later use. Unlike traditional water heaters, they don’t require electricity for heating, saving you loads of money on your electric bill.
Solar water heaters can take several forms, with each design type having its pros and cons. Some systems heat up the water directly, while others use a working fluid to collect heat and run the water through a heat exchanger. Generally speaking, water heaters come in two major design types; active and passive. Active water heaters use pumps and valves to move the water through the system, while passive water heaters rely on gravity and convection.
Active Solar Water Heaters
Active solar heating systems use a series of pumps to move the water through the heating system. They require electricity (possibly from solar panels), or some sort of fuel, to operate. Because of this, active heating systems tend to be more expensive, but they offer more design configurations and are more efficient.
Active solar water heaters can be controlled using a system of thermostats, sensors, and pumps. For example, when the water in the storage tank falls below a specific temperature, the system can be set to be automatically powered on to run the water through the solar collectors.
Active solar heating systems can be split into two categories, direct and indirect systems.
Direct solar heating systems directly heat the water that will be used in your home. The water passes through the storage collector, where it is exposed to the sun’s heat, then back into the storage tank and into your home. Direct heating systems work best in hot and sunny climates, where temperatures rarely dip below freezing.
Indirect solar heating systems use a working fluid in place of heating the water directly. The fluid medium is passed through the solar collector, where it is heated by the sun. It is then pumped through a heat exchanger, where the heat is transferred to the water supply.
The fluid medium used in indirect systems is oftentimes water mixed with an anti-freezing agent, usually propylene glycol. This prevents the water from freezing in the pipes in frigid weather. Indirect systems work best in cold climates.
Passive Solar Water Heaters
Passive solar water heaters don’t use pumps and valves to move water through the system. Instead, they rely on the principle of heat-driven convection, where hot water rises and cold water sinks, creating a natural circulation in the heating system. They don’t require electricity to operate, making them cheaper overall. But this comes with the downside of being less efficient. Passive water heaters tend to be more reliable and longer-lasting due to the lack of moving parts.
Since they don’t require many parts to build, passive solar water systems can be DIY. Passive systems come in two main designs: integral collector storage systems and thermosyphon systems.
Integral Collector Storage Systems
Integral collector storage (ICS) systems consist of a large water tank that acts as both a storage unit and solar collector. The tank is usually black and placed within a box with a transparent covering, allowing it to collect the sun’s heat and capitalize on the greenhouse effect. The walls of the box may also be outfitted with mirrors to maximize the heating capacity.
ICS systems rely on convection to circulate water through the tank. Cold water flows into the bottom of the tank through a pipe and rises to the top of the tank as it warms up. Near the top of the tank is the outflow pipe for the hot water, which transports the water to the plumbing system. One major downside to ICS systems is heat loss due to the tank not being completely insulated since it also must act as a solar collector.
Similar to the ICS system is the convection heat storage (CHS) system. This design separates the collector and storage tank, allowing for less heat loss since the storage tank can be fully insulated. The tank is placed above the solar collector, relying on convection to move the hot water up to the storage unit. Being that the CHS system is vertically oriented, the coldest water is at the lowest part of the system, and the hottest water is up top. This prevents heat loss from creating a convection current that moves water in the opposite direction.
Thermosyphon systems ditch the storage unit in favor of heating small batches of water at a time. They employ rooftop solar collectors to heat small amounts of water, usually 40 gallons at a time, which are directly connected to the tap. The water is pulled directly from the storage collectors when the hot water is turned on.
Parts of a Solar Water Heater
While solar water heaters come in various designs, there are a few essential components that are universal to all types.
The solar collector is the main feature of a solar water heater. After all, it’s the source of your hot water. The solar collector absorbs the heat from the sun, which is used to heat your water supply. The solar collector may also contain a working fluid which is then used to indirectly heat your water. Solar collectors come in several different types.
Flat Plate Collectors
Flat plate collectors consist of a thin insulated box with a transparent cover on the face, usually made of hard plastic. Under the transparent covering are several tubes through which the water or fluid medium flows to collect heat, placed atop an absorber plate. The entirety of the flat plate is black to maximize heat absorption, including the outer covering, absorber plate, and inner tube system. Flat plate collectors often resemble PV solar panels.
Evacuated Tube Collectors
Evacuated tube collectors consist of a unit containing several tubular absorbers encased within vacuum-sealed glass tubes. The vacuum seal reduces the convection and conduction of the collected heat into the ambient air, significantly increasing efficiency. Like flat plate collectors, the tubes are placed onto black absorber sheets to maximize heat absorption. Because they require two layers of glass, they are often treated with an anti-reflective coating to maximize absorption.
ICS Tank Collectors
ICS systems rely on a storage tank placed within an absorption box as a solar collector. The tank is usually tubular and laid out horizontally to maximize the surface area exposed to the sun. The box is similar to a bulky flat plate collector, featuring a transparent hard plastic covering with insulated sides and an absorption plate on the bottom. The interior of the box may also have several mirrors to aid in heat collection.
Water Storage Tanks
Solar heating systems often employ storage units if the water is not meant for immediate use. The size of the tank varies, but they are always insulated to help retain heat. The placement of the storage tank depends on whether the heating system is active or passive. Active systems allow for the storage tank to be placed below the collector, as the flow of water can be controlled via pumps. Passive systems usually require that the storage tank be above the collector, as this allows the hot water to rise into the tank by convection.
Heat Exchangers & Working Fluid
Indirect solar water heaters use a working fluid to collect the sun’s heat, then transfer that heat to the water supply using a heat exchanger. The working fluid is generally a mix of water and propylene glycol, which serves as anti-freeze.
The heat exchangers transfer the heat from the working fluid to the domestic water supply. Most heat exchangers consist of a coil-in tube where the two tubes make contact. One tube surrounds the other, and heat is exchanged. The heated water is then pumped towards the storage tank, and the working fluid is pumped back towards the solar collector.
Pumps, Controllers, & Sensors
Active systems require one or more pumps and controllers to function. The pump forces the water towards the rooftop solar collector and back down into the storage tanks. This allows the tank to be installed on the ground, as convection currents are not necessary to move the water. The pumps may also be used to move the working fluid through the loop system.
Controllers and sensors work in tandem to measure the temperature and water levels in the solar collector and storage system. These sensors send signals to the controller, which tell the pump to turn on or off depending on the need for water.
Solar water heaters aren’t perfect. As with PV panels, due to their reliance on the sun they’re at the mercy of the weather, both in terms of their ability to function and their exposure to the elements.
Solar water heaters can still work in cold climates as long as there’s ample sunlight for the solar collector to absorb heat. The problems arise when frigid temperatures threaten to freeze the water pipes solid and damage your system. If you live in a cold climate, an active indirect system is your best bet, as the working fluid is treated with anti-freeze and will take the brunt of the cold temperatures.
You can take other measures to reduce the risk of frozen pipes by installing insulation, using rubber silicone pipes to accommodate freezing, and draining the system when cold weather is expected. Drainback systems, where the water drains out of the system when the pump is off, can help mitigate freezing.
Overheating & Heat Loss
Controlling the heat within the solar heating system can prove to be a challenge. When the hot water hasn’t been used in a few days, heat can accumulate in the system, bringing the water to dangerously high temperatures and causing damage to the system. Direct systems can be set to deliberately circulate water through the collector at night to dissipate the extra heat. Drainback systems can also help protect against overheating.
All solar heating designs suffer some form of heat loss. ICS systems and flat plate collectors remain uninsulated around the acrylic cover, radiating heat back into the environment. Indirect systems suffer from heat loss at the exchange. While steps can be taken to mitigate heat loss, no design can completely prevent it from happening.
Some may choose to install a backup water heating system for cloudy days and cold nights. This usually consists of a traditional heating system powered by electricity or gas. In these cases, the water heater works on a hybrid system, where solar heating is used when the sun is available.
Active solar water heating requires a power supply to work the pumps, sensors, and controllers. Compared to a traditional water heater, the amount of energy necessary to work the system is small. Homeowners with active systems can connect their pumps to their domestic power supply. One can also install a few PV panels for a completely carbon-neutral set-up.
Solar water heaters require maintenance to remain functional. For the most part, it takes as much care as your plumbing system. Pipes, valves, and pumps all need to be inspected every so often. Active systems require more maintenance than passive systems, as they have more moving parts. Climate also plays a role in the frequency of upkeep. If you live in a dry climate, you will need to constantly wash off the dust from your solar collector. If you live in a cold climate, you will need to take care not to leave water in the system during freezing weather.
Choosing the Best Solar Water Heater for Your Needs
Solar water heaters come in several design types, and the best design for your home depends on your specific needs. A few of the factors to be considered when deciding on, or designing, a solar water heating system are listed below.
-Local climate and solar irradiance
-Hot water use, both in terms of total monthly volume and peak use times
-Type of dwelling/structure and building codes
-Energy efficiency of current water heating system
Like installing a PV system, if you decide to hire a contractor to install your solar water heater, do the research to make sure you are hiring the best contracting company. A good solar installer will have years of experience, good customer service, won’t be too pushy, and will offer great warranties that include maintenance and repairs.
Solar water heaters can save you tons of money on your electric bill. Hot water is a huge source of energy, so why not harness the power of the sun? But there are a lot of things to consider when planning to install a solar water heater. There are many different designs, and choosing the best one for your needs depends on several factors. You can choose to hire a solar contractor to install one for you, or if you’re the engineering type, you can build and install one yourself. Once installed, you can use your solar water heater to heat up your pool, take hot showers, or even for space heating. If you want to go completely carbon-free, you can even install solar panels to power the pumps and mechanisms in your water heating system. The possibilities are endless, so what are you waiting for? Get your solar water heater installed today!
Frequently Asked Questions
A solar water heater uses heat from the sun to heat up your domestic water supply.
Solar collectors are placed on the roof of your home, and the water is run through the solar collector. The solar collector absorbs heat, heating up the water, which is then transported back towards your home.
Solar water heaters come in two main types; active and passive. Active systems use pumps and valves to transport water through the system and so require power to operate. Passive systems rely on convection currents resulting from heat absorption to move water through the system. They do not need power to operate.
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