According to the National Renewable Energy Laboratory, replacing an electric water heater with a solar heater can offset the equivalent of 40% to 100% of the carbon dioxide emissions of a modern passenger car. The DOE estimates that solar hot water can reduce hot water heating bills by 50%-80%.

A prerequisite for installing a system for a brownstone is to have an unshaded roof from which you can orient south-facing solar panels – note I did not say a “south-facing roof” because, as brownstone roofs are often relatively flat or only slightly sloped, it’s not really relevant whether the roof slope towards the south as long as the roof has a large enough unshaded area to position panels in a southerly orientation. Consult the DOE Consumer Guide to Siting Your Solar Water Heating System's Collector for guidelines on orientation and tilt.

The volume of hot water that a solar thermal system produces depends on the size of the unit. A general rule of thumb is that 20 SF of solar collector area (say panels stretching over a 3-foot by 6-to-7 foot area) will provide enough hot water for 2 people, and in the northeast for each additional person you would need an additional 12 to 14 SF of collector area. Given the limited size of brownstone roofs it may not be possible to have a system large enough to supply all your hot water needs, which vary seasonally, but it can certainly make a dent and lighten the load on a conventional or tankless back-up heater.

Solar thermal basic components:
Basically the systems consist of solar heat collection plates and a storage tank. The collection plates come in two types: flat plate panels and evacuated tube solar collectors. Flat plate panels are weatherproofed boxes holding a dark heat absorber plate which can typically heat water to up to 160 degrees. Evacuated tube solar collectors contain rows of solar heat absorber tubes housed within outer tubes and the air is removed (“evacuated”) from the space between the tubes which eliminates conductive and convective heat loss. These systems can heat water from 170 – 350 degrees but are more expensive than flat plate panels and are typically used in very cold climates or commercial applications, although they are cropping up more and more in residential projects so discuss this with your solar congtractor.

Either kind of heating system can be “active” which relies on pumps to move the water through the system, or “passive” which relies on gravity and physics, in particular the natural tendency of hot water to rise, to move the water through the system.

Active, or Forced circulation, solar systems
This type of system uses a pump to circulate water or other fluid from the solar collector point where it is heated, to a storage tank. There are two types of active systems: Direct circulation systems pump the actual household water through the heat collectors then into the home plumbing system for use, but can only be used in mild climates where there is no risk of the water pipes freezing; thus this system would not work in NYC.  Indirect circulation systems circulate either a non-freezing, heat-transfer fluid, or air, through the collectors and a heat exchanger. This heats the water that then flows into the home. The indirect circulation system would work in the northeast climate zone.

A liquid-to-liquid active system passes the heat transfer fluid through the solar collector, absorbing heat in the process and then passing the heated liquid through a heat exchanger to transfer this heat to the domestic hot water. The hot water and the heat transfer fluid never mix because the heat transfer liquid may be toxic; the two are separated by single- or double-wall piping. Special care should be taken when choosing the heat transfer fluid depending on your climate zone, but for NYC and anywhere where the temperature dips below 42ºF the one viable choice is to use an anti-freeze liquid, otherwise you will need to drain the pipes whenever there is a risk of the temperature dropping below 42ºF, a high-maintenance non-viable approach as it decommissions your hot water heater during the winter season. Certain anti-freezes, such as ethylene glycol, require double walled piping because they are more toxic than other alternatives, such as a food-grade propylene glycol/water mixture that is certified as non-toxic and therefore can be used with single-walled piping. Anti-freeze fluids need to be checked annually for pH value, freezing point and adjustments made to maintain their stability. Silicones may be another option because they have a very low freezing point and a high boiling point but they are viscous and require more energy to pump and they leak easily even through microscopic holes.

U.S. Department of Energy

Passive Solar Systems
Passive solar systems do not use pumps or any mechanical means to circulate water or heat transfer fluid. There are three basic types of passive systems: integral collector-storage or “batch” system, thermosiphon system, and self-pumping system.

An integral collector-storage (ICS) system or “batch” system combines the collector and storage tank into one unit by, essentially, placing a storage tank inside an insulated box with its dark, heat-absorbing side facing the sun. These systems are also used to pre-heat water that is then passed through a conventional water heater which consequently needs to use less energy to further heat the already-warmed water; however, the tank must be drained in cold climates so a batch system is not viable for NYC.

A thermosiphon system has a separate storage tank located above the solar collector. As water heats up in the collector tank, thermodynamics will cause this heated water to rise into the storage tank. You likely realize that this means all the water storage is on the roof, so careful attention must be paid to structural support for heavy water tanks.

U.S. Department of Energy.

A self-pumping system uses a “phase change”, such as the change of a liquid to vapor or other passive means, to cause the water or fluid in the collector to circulate and transport heat from the collector to the storage unit.

The need for a back-up system.
Keep in mind that the sun isn’t always shining, so it is necessary to design any solar hot water system with a back-up hot water heater, either a conventional storage tank or an in-line/tankless heater(s).

Understanding Solar Water Heater Energy Efficiency
For a solar water heating system, the solar energy factor (SEF) and solar fraction (SF) are used to determine a system’s thermal performance. The SEF is analogous to the Energy Factor (EF) rating given to conventional hot water heaters. The Solar Rating and Certification Corporation (SRCC), a non–profit organization that tests and rates solar heating systems, includes on its website a Directory of SRCC Certified Solar Water Heating Systems Ratings February 17, 2008.   In the SEF calculation, the energy delivered by the system is divided by the electrical or gas energy put into the system; consequently, the type of back-up hot water heating system that is being used will have a large impact on the performance of the system as a whole.

The SF calculation involves the comparison of the SEF and EF; however, in the marketplace the term is used to refer to two different calculations: one calculation/definition yields a result that describes the portion of the total conventional hot water heating load (delivered energy and tank standby losses) provided by solar energy (this is the definition and calculation used by the SRCC); the alternate definition/calculation yields a result that states the portion of the total water heating load (losses are NOT included) provided by solar energy, and this alternate method of calculating solar fraction will yield higher solar fractions. Consequently, ensure when comparing system results that the same calculation was used for each unit or you will be comparing apples to oranges. The higher the solar fraction, the greater the solar contribution to water heating, which reduces the energy required by the backup water heater. The solar fraction varies from 0 to 1.0. Typical solar factors are 0.5–0.75.

Now the big question: is solar hot water heating worth it economically? Consult the Department of Energy’s Consumer’s Guide Estimating a Solar Water Heater System's Cost for an excellent analysis and explanation of how to determine the annual operating costs of a solar thermal system for your home, and how to calculate the pay-back period.  The SRCCs ratings document in the link above also includes information on how to compare the costs of systems along with different back-up systems.  Also check back to our 168 Clinton St. blog where we will apply the analysis for brownstone use. 

Federal, state, local and utility incentive programs exist to help homeowners afford the costs of increasing hot water energy efficiency. For a complete discussion of incentive programs see our Article on Incentive Programs for Green Renovation