Unexpected Benefits

The thermodynamic advantages of combining radiant heat with solar energy are a little subtle. Nevertheless, the layman can appreciate these advantages with a little study. You will see here how the combination is a great improvement in solar heating that offers much greater efficiency with lower cost.

The sun is after all, "the mother of all radiant heating systems" and its partnership with mechanical, earthly underfloor radiant systems would seem to be a natural thing. There are many benefits to this combination, and this appreciation has taken the practice of solar heating design to an entirely new level.

Active Approach The great advantage of active design is that the solar panels receive solar energy during the day but do not lose heat at night as the passive aperture does. Active systems also have tight temperature control. But active systems are expensive, somewhat inefficient with all of their heat exchangers and the storage element is large and obtrusive.

A typical "active" system

Passive Approach The advantages of passive design are that they are lower in cost and they use components of the architecture to accomplish solar heating goals. It is more natural and non mechanical. However, passive designs are limited by the loss of heat at night and on cloudy days and they require the occupant to accept wider temperature swings. Also, the need for domestic hot water is not simple to address and this is important.

A typical "passive" system

The Hybrid Approach

It is possible to make improvements in dogs, vegetables, flowers and solar heating systems with a "hybrid" or combination approach. The goal is to combine the relative advantages of each school of thought and leave the disadvantages behind.

We must note here that if there are significant benefits of hybrid design, the partisans of the underlying schools of thought may be the last to know. Indeed, we may have another example of how education can be an obstacle to innovative thought.

Nevertheless, if we were to come up with a combination or "hybrid" solar heating system, it might look something like this. Active solar collectors harvest the solar energy and then inject it into a radiant panel slab. The heat is then stored and released in a passive slab. The heat is then stored and released in a passive manner. We can collect solar energy during the day, but we will not lose energy at night. We can construct a very energy efficient building so that we do not need a lot of energy and we make domestic hot water with any excess. The project will be simple and very efficient. There will be an excellent return on the solar investment.

With these principles in mind we turn our attention to the "unexpected storage potential" that careful research has informed us of.

The happy confluence of solar circumstances Under typical winter weather patterns, very cold temperatures are accompanied by bright sunny conditions while cloudy conditions are accompanied by warmer weather. In other words, bright solar energy tends to arrive just when the building needs it most. This phenomenon greatly helps the usefulness of solar heating systems. The graph at right illustrates how a radiant storage mass can buffer extremes in temperature and solar availability without complicated controls.

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Higher heat storage potential The temperatures within a building under heating load are not uniform. The floor tends to be the coldest part of the building because of radiant losses to windows and cold walls and also due to "drain down" of cold air from drafts and colder surfaces. Indeed the floor will tend to be 5-15°F lower than the rest of the building under steady state conditions.

But if the floor is going to fully heat the building, it must be 5-10°F warmer than the rest of the building. That means that we can simply dump a considerable amount of solar heat into the floor and the floor will absorb it without changing the overall temperature reading of the building.

If we accept a broader "hysteresis" between minimum acceptable temperatures and maximum acceptable temperatures, in the passive manner, the storage potential of the floor increases. A temperature setback at night increases the storage potential of the floor further.

We knew that the hybrid solar heating system would be simple and efficient. What we did not fully expect was that a very high solar heating fraction could be achieved in this very simple manner. This serendipity has pointed the way to solar heating designs that are a significant improvement over the past.

The Department of Energy report concluded as follows:

A significant advantage of the Solar Option 1 system is that it is capable of meeting nearly all of a residential type building's heating needs, even in quite challenging climates. Another advantage is exceptionally high efficiency that very low operating temperatures entail. "Substantial improvements were noted in system efficiency, overall performance, initial cost and architectural flexibility. An increase in collector efficiency translates into fewer solar panels, lowered costs, and easier design integration into accepted building styles. A large thermal mass, integrated within the building's structure provides prolonged solar storage, radiant comfort and further lowered costs."

- Report to US Department of Energy
1983 (DOE/CE15140-T

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"If we knew exactly what we were doing it would not be called research"

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The home is an exciting blend of active (electrically operated) and passive (self-powered) design strategies - a hybrid solar home - that provides essentially 100 percent solar space and water heating in a severe 8000-degree-day climate. The solar system is almost unbelievably simple, and offers a unique combination of reliability, low cost, ease of installation, architectural flexibility, comfort control, and best of all, truly outstanding thermal efficiency. In many ways, it leaves conventional active and passive solar systems in the dust.