Tuesday, June 28, 2011

Ways to use solar water heating in a home

I love to talk and write about solar technologies.  My goal has always been to encourage all things solar because I think awareness of one type of solar system will benefit all the others.  Solar PV is the darling of the clean energy world--for good reason.  It produces versatile energy in a clean, quiet, safe, and predictable way.  The major downsides are its current cost and efficiency.  While these problems will be overcome with time, solar water heating is ready today.  It's still fun for me to see a home making 130 degree F water just from the sun on a cold March afternoon in Chicago. While my primary focus is on solar domestic water heating, I wanted to expand on some other ways a solar water heating systems could be used in a normal US home.

Solar Domestic Water Heating
The most simple way to harness the power of the sun in a home (after daylighting and applying passive solar design principles) is to heat up domestic (drinking) water with a solar water heating system.  This technology is well-established and easily adaptable to almost any home or building with a hot water demand.  Solar domestic water heaters are sized to reduce the water heating bill of any home in the US by 60-80%.  A typical home would need about 2 solar panels (50 - 80 sq. ft.) and about an 80 gallon storage tank.  Any solar system of this type will have some sort of back up water heating component to ensure that the residents have a steady supply of hot water despite the solar conditions of a particular day.

Solar Space Heating
If a homeowner is looking to squeeze a little more heat out of a solar water heating system and try to contribute some of this free energy to heating the home, there are a couple of ways to accomplish this. Not to be confused with a solar air heater, I'm still talking about an automatic system that heats up a liquid for use in the house.

Option 1:
Homes with an existing forced air furnace could install a solar water heating system sized to heat nearly 100% of the domestic water heating demand plus make a contribution to space heating (~25% or so).  A system of this sort would have about 5-6 solar collectors (150 - 240 sq. ft.) and about 200 gallons of storage tank volume.  This solar system would operate just as in the diagram above but have a second heat exchanger for transmitting solar heat into the return of the forced air furnace.  The second solar heat exchanger would pre-heat the air returning to the furnace so that it has less of a temperature rise to meet the desired room temperature.
Water-to-Air heat exchanger that would go in the air return side of a forced air furnace
Option 2:
Solar water heating systems could also deliver space heat through the use of a fan convector. A comparably sized system to #1 but without integrating the space heating delivery system to the furnace.  The homeowner could install a system which would automatically divert solar heated fluid through a fan convector(s) strategically placed in the home.  In this scenario, heat from the fan convector would warm the space as it becomes available from the solar system.  This contribution would help keep the thermostat from activating the primary space heating system as often.  A good application for this option might be for a vacation home that needs to be kept warm enough to prevent the pipes from freezing when not being occupied.
Myson fan convector unit on the left. Probably VELUX roof windows above.
Option 3:
Some homes have a hydronic heating system where medium temperature (140 degrees F) water circulates between a water heating unit and various zones throughout a house.  This heating system lends itself to a nice merger with a solar water heater.  As the hydronic fluid returns from a zone in the house depleted of heat, a heat exchanger from the solar storage tank could add heat to the fluid before it reaches the traditional heater.  This is one of the most efficient ways to deliver heat from a solar heated fluid to a living space.  These solar systems could be sized to be rather large (8-10+ collectors) depending on the space heating goals the home may have.
Tubes under the floor deliver heat to the room in a hydronic heating system
For any of these options, this style of heating system isn't usually the best primary space heater for a home since the winter months (when you want heat) typically correlate to shorter days and thus less solar energy to be harvested. Like with a solar domestic water heater, however, a solar space heating system will reduce the number of BTUs the traditional heater will have to deliver to the house even during the winter months.

Solar High Mass Systems
In this variety of space heating using solar energy, a solar water heating system heats up a large mass (many, many tons) of sand under a home which then slowly releases the heat over the winter months.  Owners of these systems begin to direct the solar heat to the sand mass which extracts the heat through a matrix of tubes embedded in the sand.  Many homes in cold climates are accomplishing 75%+ of their space heating needs through this style of solar water heating system.  Not all that complicated or that expensive, Solar High Mass systems will require a bit more homeowner involvement to make them work effectively with their lifestyle.  This also would need to be designed into a new home from the start since so much sand or other high mass material would need to be under the living space.  The Artha Sustainable Living Center is a great resource for more information on these systems.

Solar Assisted Ground Source Heat Pumps
A ground source heat pump (GSHP), aka geothermal, is a device that heats and cools a building by moving heat to and from the living space to a series of pipes in the ground.  Additionally, a GSHP can contribute to the domestic water heating needs of a home as well.  These systems function in a very similar way that a kitchen refrigerator does except the ground is the heat sink in the cooling cycle versus ambient air as with the refrigerator.
Imagine an array of 3-5 solar collectors on the south facing roof to integrate with the GSHP

A rather new implementation of a GSHP adds an array of solar water heating panels to the mix.  This Solar Assisted GSHP offers a few advantages over a standard GSHP:
  1. The solar collectors can add heat to the ground all summer to allow the GSHP to be more effective at space heating in the winter.
  2. Solar panels can pre-heat domestic water in the summer so that the heat pump may not need to function at all to provide domestic hot water when the weather is temperate.
  3. Added heat can come from the solar collectors in the winter to augment the space heating cycle.
  4. During the spring and fall, the heating load may be low enough that the solar collectors could provide enough space heat so that the heat pump would not need to operate.
GSHPs slowly deplete the ground heat immediately surrounding the bore wells over the life of the system.  For this reason, many green building codes don't classify this technology as "sustainable" or "renewable."  Coupling a solar array to the GSHP is a way to make this excellent technology truly sustainable.

The solution to our energy problems rises in the east every morning.


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