Solar Water Heating System
at the
Campus Center for Appropriate Technology
by Andy Karnitz
Introduction
American homes, schools, and businesses have been using solar energy to meet their water heating demands for over 100 years. Water heating accounts for 15% of household energy use, 90% of which can be obtained from the sun from May to October. Solar water heating systems utilize the energy of the sun by heating either water or a heat transfer fluid in a collectors. These types of systems typically reduce the need for conventional water heating by two-thirds. There are many different variations of solar water heating systems and various components that can be added to a system to increase efficiency. Figure1 (below) shows the basics of a solar water heating system.
Figure 1: Basic Solar Water Heating System (courtesy solaror.org)
Direct Pumped System
The direct pumped system, Figure 2, has one or more solar energy collectors installed on the roof and a storage tank somewhere below. A pump circulates the water from the tank up to the collector and back again. This is called a direct (or open loop) system because the sun's heat is transferred directly to the water circulating through the collector tubing and storage tank; no anti-freeze solution or heat exchanger is involved. This system typically has a differential controller that senses temperature differences between water leaving the solar collector and the coldest water in the storage tank. When the water in the collector is about 15-20ƒ F warmer than the water in the tank, the pump is turned on by the controller. When the temperature difference drops to about 3-5ƒ F, the pump is turned off.
Figure 2: PV circulated direct pumped system (courtesy Florida Solar Energy Center)
Indirect Pumped System
The indirect pumped system, Figure 3, design is common in northern climates, where freezing weather occurs more frequently. An antifreeze solution circulates through the collector, and a heat exchanger transfers the heat from the antifreeze solution to the tank water. In this system a heat transfer solution is pumped through the collector in a closed loop. The loop includes the collector, connecting piping, the pump, an expansion tank and a heat exchanger. A heat exchanger coil in the lower half of the storage tank transfers heat from the heat transfer solution to the water in the solar storage tank. An alternative of this design is to wrap the heat exchanger around the tank. This keeps it from contact with potable water.
Figure 3: Indirect pumped system (courtesy Florida Solar Energy Center)
CCAT's System
CCAT's indirect pumped solar water heating
system consists of solar collectors, a storage tank and a circulation system.
In this system, a glycol based heat transfer fluid circulates
between the flat-plate solar collectors and the storage tank. Flat plate
collectors are the most common type of collector. These collectors are insulated, weather-proof boxes containing a dark plate that
absorbs solar energy covered by a transparent or translucent glazing. The heat
from the fluid then is given to the water in the storage tank via a heat
exchanger. A small
photovoltaic panel on the roof runs a DC pump which recirculates the fluid when
there is enough solar energy to power the pump. The theory behind this
setup is that if there is enough solar energy to power the pump, then there is
enough solar energy to heat the fluid. CCAT goes one step further to avoid
wasting resources. They use an on-demand, or flash, water heater to heat
small amounts of water when it is needed, instead of continuously heating like
conventional water heating systems This heater adds the energy that the sun
cannot always provide, because sometimes
you need hot water when the sun isn't shining. Using solar energy to heat water is one more way
that CCAT demonstrates that living lightly upon Earth is neither difficult nor burdensome.
For more information on harnessing the power of the sun check these links:
Solar Energy Association of Oregon - www.solaror.org
U.S. Department of Energy - Energy Efficiency and Renewable Energy Network (EREN) - www.eren.doe.gov
Home Power Magazine - www.homepower.com
Solar Water Heating Consumer Guide - www.solarexpert.com/Heatdhw.html
Andy Karnitz created this page for CCAT as part of the requirements for Engineering 115 at Humboldt State University, Spring 2002