Active vs. Passive Solar Water Heaters

Solar water heaters are one of the most energy-efficient ways to provide hot water for a home.

Unlike other water heating systems, the energy source they rely on — sunlight — costs nothing and is emissions-free. They can also work in a range of climates and for a variety of household sizes.

Before someone commits to a home solar water heater, however, they’ll need to decide if they want an active or passive system.

While these two types of solar water heaters have the same essential components, there are a few key differences between them that change how they perform and how much they’ll cost.

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    Passive Solar Water Heaters

    All solar systems have at least the same two parts — an insulated water storage tank and one or more solar collector panels that absorb solar radiation.

    Passive solar water heaters, on the other hand, rely on a combination of gravity, water pressure, and convection. Convection is the tendency of hot water to rise above cold water.

    There are several types of passive solar water heaters, with two being the most popular. The simplest passive systems are integral collector storage systems. These use collectors to pre-heat incoming water before they flow into a home’s storage tank. Because these systems are so simple — no moving parts and few components in general — they tend to be very low-maintenance, meaning few repairs and a lower overall cost of ownership.

    Common issues, like damage to the collector and mineral build-up in the system’s pipes due to hard water, are also typically easy to resolve.

    More complex passive systems, like a thermosyphon system, are also an option.

    A thermosyphon (or thermosiphon) system is made up of a solar collector and a water storage tank that’s positioned higher than the collector.

    The collector heats water towards the bottom of the tank. Water heated by the collector rises to the top of the tank, and colder water returns to the bottom. This water comes back into contact with the collector and heats up. This cycle generates enough hot water for a home.

    These systems typically use flat-plate collectors — which consist of a dark absorber plate in an insulated and weatherproof box — or evacuated-tube solar collectors, which use transparent glass tubes to collect solar radiation.

    Like the integral collector system, the thermosyphon system has no moving parts, which tends to make maintenance both easier and cheaper.

    Passive solar heaters can be vulnerable to sub-zero temperatures, however. Piping that runs through non-conditioned or insulated spaces are liable to freeze when temperatures drop below 0 degrees Fahrenheit.

    Pipe insulation and certain pipe materials can help prevent burst pipes. PEX, for example, is more flexible and tends to hold up to pressure from freezing better than copper.

    Active Solar Water Heaters

    Like passive water heaters, active water heaters use solar collectors to warm water for household use. These collectors are typically flat-plate collectors or evacuated-tube collectors, as in thermosyphon systems.

    What makes active solar water heaters different from passive heaters is that they have active components — pumps and electronic controls that move liquid around the system.

    These electronic pumps are typically small and require little energy because they’re not moving a large amount of liquid through the heating system. However, they will require some power to operate, unlike passive systems. This means that unless a home has a dedicated backup generator, the system will be unable to effectively heat water during a power outage.

    There are two types of active solar water heater systems — direct and indirect circulation systems.

    Direct circulation systems use the pump to move water from the storage tank to the collector. These systems are most effective in areas where temperatures don’t dip below freezing. 

    In areas that regularly experience sub-zero temperatures, they’ll need to include a recirculation system that keeps water moving and prevents freezing. These systems can increase the power draw of an active solar water heater.

    Indirect circulation systems circulate a heat transfer fluid (HTF) — often a mixture of antifreeze and water — between the collectors and a heat exchanger attached to the tank. This refrigerant heats the water in the storage tank and ensures that the system’s pipes don’t freeze in sub-zero temperatures.

    Because antifreeze mixed with water can deteriorate over time, the HTF will need to be replaced every few years. While this task is simple, a technician may be needed to replace the HTF safely.

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    Using Solar Water Heaters With Conventional Heating Systems

    Some solar water heater systems can be used in conjunction with conventional systems — providing a middle-ground option for homeowners who want to make their home more eco-friendly but don’t want to give up on the reliability of a conventional heater.

    Another option is the use of electric or gas backup heaters. These heaters only activate when the solar water heater system isn’t producing enough heated water for the house, or manually as needed.

    Because a solar water heater can supply between 80% and 90% of a home’s hot water needs, the electric backup would only need to be used rarely, when there’s little available sunlight for long periods of time.

    These systems perform similarly to conventional water heaters, but tend to last longer and consume less energy.

    The lower demand for hot water put on these conventional systems means less strain on the heating elements and other parts. While some maintenance will certainly be necessary, they may need less care than a conventional system without a solar heater attached.

    The drawback of this kind of system is in the up-front installation costs. Coordinating the two systems means a homeowner will need to pay for the installation of both the conventional heater and solar heater as well as the piping and other systems that link the two together.

    Depending on the particular configuration of a home and a homeowner’s needs, a home water heating system with an electric backup can be more expensive than either the conventional system or the solar heater system on their own.

    Cost and Benefits for Active and Passive Heaters

    Combined with other green tech, like solar panels and rain barrels, active solar water systems can be both extremely eco-friendly and effective at heating a home’s water.

    However, they are more complex than passive systems — meaning higher installation costs and more complex maintenance. The pumps necessary to keep water flowing will also need some electricity, meaning the system may not be emissions-free in a home that draws energy from the grid.

    Passive systems are typically cheaper, but they also take longer to heat water than an active system. As a result, they may be less effective for homes that demand large amounts of hot water over a sustained period of time.

    Active solar systems and indirect solar heaters especially also tend to fare better in colder climates. 

    Any solar heater system can be paired with a conventional heater that serves as a backup. Simple passive heaters can also pre-warm water for a conventional heater, allowing a homeowner to cut down on water heating costs without completely relying on their solar heater system.

    In general, the more hot water a household uses, the quicker a solar water heater system will pay for itself. If a home uses a large amount of hot water, they’re more likely to benefit from more expensive active heating systems. Homes with less need for hot water may find that passive systems are both good enough to meet their needs and a more economical option.

    The effectiveness of any solar water heater will also depend on how much direct sunlight a home gets. If there is no good location for solar collectors on the property, they may be much less efficient.

    While solar heaters can be used for home water heating in any part of the country, systems in certain locations may require collectors with more surface area, or greater care put into collector location and angle.

    Key Differences Between Active and Passive Solar Water Heaters

    Solar water heaters are one of the most efficient and eco-friendly ways that a person can heat water for home use. Both passive and active solar heating systems can be effective, but different households may need different system types, depending on their needs and location.

    Larger homes with a high demand for hot water will likely need a slightly more complex and costly active water heater. 

    If the home is in an area where temperatures regularly dip below freezing in the winter, they’ll also need to invest in an indirect active system that uses some kind of heat transfer fluid, or a direct active system that recirculates water.

    For smaller households with less need for hot water, simpler passive systems may be effective. 

    If a household wants to incorporate a solar water heater without ditching their conventional water heating setup, they may also use an integral collector system, which can preheat water for use in a conventional heater.

    Author bio:
    Jane works as an environmental and energy writer. She is also the founder and editor-in-chief of Environment.co.