Sun Showers

By Stu Weinreb

Mass Audubon, New England’s largest conservation organization, works to protect the natural environment of Massachusetts for people and wildlife. In addition to a large membership and a statewide network of 43 wildlife sanctuaries, the organization operates an overnight summer camp--Wildwood--in Rindge, N.H., with programs for campers ages 9 to 17, and for families.

Since its environmental mission has led to an understanding and belief in the value of green buildings as an important aspect of protecting the environment, it’s no wonder the organization began exploring ways for the camp to be more sustainable. Although most of the focus for the sanctuaries has been on photovoltaic arrays and the generation of electricity from solar energy, this was not particularly economical for the seasonal camp facility.

However, Wildwood does have a significant hot-water demand due to camper and staff showers, as well as washing clothes. With this in mind, the organization decided to research the idea of a solar-thermal domestic hot-water system to reduce the reliance on fossil fuel (propane) to heat domestic hot water in a relatively new shower house. Mass Audubon worked with Littlefoot Energy--a design/build firm--to help understand the different types of solar-thermal systems available, and to match a system to its needs.

Two types of solar-thermal systems were considered:

1. Flat-plate
2. Thermo-siphon.

Flat-Plate System
A flat-plate system uses solar collectors that are filled with a heat-transfer fluid (glycol mixture) that is heated in the roof-mounted collectors, and is pumped to a heat exchanger located inside a “buffer tank” in the basement of the building. The buffer tank provides heat storage in the form of hot water that is transferred to the standard domestic hot-water tank as needed. This type of system uses pumps and pump controls to regulate the flow of glycol to and from the collectors, and to and from the buffer tank to the domestic hot-water tank.

A flat-plate system is great for year-round use as it is designed to operate continuously and typically is not drained seasonally. Since Wildwood has a relatively low program usage beyond the camp season, it was decided that a flat-plate system was not a good fit. Instead, attention was focused on a thermo-siphon system.

Thermo-Siphon System
“Thermo-siphon” refers to a method of passive heat exchange based on convection. This system requires no water pumps or controls. Instead, it operates by exploiting a natural property of the water that fills the panels. As the sun hits the solar hot-water panels, the water within the panels begins to heat up. The warm water has a lower density than the cold water, and begins to rise. Above each panel is a water-storage tank, which acts as a “battery” for the solar energy. Hot water rises to the tank, and cold water diffuses towards the panels. This cycle continues until the tank reaches a certain temperature, or until there is no more solar-heating energy available. Existing domestic hot-water tanks, located inside the building, draw hot water from the solar hot-water tanks, instead of heating water using the existing propane boiler. If there’s insufficient hot water in the solar hot-water tanks, the existing propane boiler makes up the difference, so no one has to take a cold shower.

Paying Off
In June and July 2009, Wildwood’s solar thermal-siphon system--comprised of 12 4-foot by 10-foot panels--was installed. Each pair of panels is connected to the unit; the system is mounted on four rows of galvanized racking designed specifically for metal roofing. A web-based monitoring system tracks the operation of the system.

As of August 2010, the camp hadn’t used the system for a full season. However, some general observations were made--the temperature of the water coming from the system can be viewed easily, and water coming into the water-heater tank in the building has been estimated at 85 F to 90 F. This is a sharp contrast to the more typical 45 F to 50 F water that would otherwise enter the tanks. Also, the propane-fired hot-water heaters are fired less often, and for a shorter duration.

The solar-thermal system was designed to offset approximately 60 percent of the typical fossil-fuel usage in the shower house. The hope is to confirm this reduction in propane usage, and declare that the sun’s energy has eliminated the need for approximately 3,000 cubic feet of propane per year. Additionally, it will reduce carbon-dioxide emissions by over 10,000 pounds per year.

The total cost of the system was just over $48,000. It will pay for itself within 10 years, after which the camp will provide hot water at a minimal cost for the balance of the system’s lifespan, estimated to be 25 years.

Beyond the very real environmental benefits inherent in a solar-thermal system, another advantage of the highly visible and frequently used facility is to introduce an important message about appropriate technology. Staff members developed an interpretive panel that explains the system in easy-to-understand terms so that the importance of green building can be passed along to campers, seasonal staff and visitors.

Stu Weinreb is the Director of Capital Assets & Planning at Mass Audubon. He can be reached at sweinreb@massaudubon.org.