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The Benefits of Hybrid Solar Design


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

The sun is "the mother of all radiant heating systems" and its partnership with mechanical, earthly radiant heating systems is a good thing. There are many benefits to this combination, and an appreciation of these benefits can take the practice of solar heating design to an entirely new level.



THE PURELY 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 tend to be expensive, and multiple heat exchangers lower efficiency. The storage element is large, expensive and obtrusive.
A typical active system
A typical "active" system

THE PURELY 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. They require the occupant to accept wider temperature swings than many are accustomed to. Also, the need for domestic hot water is not simple to address and this is important.
A Typical Passive System
A typical "passive" system


THE HYBRID APPROACH

We can make improvements in vegetables, automobiles 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.

If we do our work well, we can design a hybrid solar heating system with higher efficiency, lower cost, better overall performance, fewer solar collectors and greater architectural flexibility than earlier designs


Imagination is more important than knowledge
- A Einstein

But we must note here that if there are benefits to a hybrid design, human nature may dictate that the partisans of the underlying schools of thought will have difficulty accepting the fact.


If we were to come up with a combination or "hybrid" solar heating system, it might look something like this.

Active solar collectors would harvest the solar energy and then inject it into the middle of a massive radiant panel slab. The massive radiant slab should incur little or no extra cost.
The heat is then stored and released by the slab in a passive manner. We can collect solar energy during the day, but we will not lose energy at night as passive apertures do.
Then, we can construct a very efficient building without a lot of unnecessary glass so that we do not need a lot of heating energy in the first place. Then we will make domestic hot water with any excess solar heating energy.

The project will be simple, efficient and reasonable in cost. There will be an excellent return on the solar investment. The solar energy supplement will be applicable to many, many slab on grade projects, without adding a lot of complexity.


Solar collector


IN 1983, A RESIDENTIAL SIZED BUILDING WAS CONSTRUCTED ACCORDING TO HYBRID DESIGN PRINCIPLES AND THEN CAREFULLY MONITORED FOR PERFORMANCE.

THERE WERE SOME INTERESTING SURPRISES.

Solar Barn

HELP FROM THE WEATHER

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.

What some have called "the happy confluence of solar circumstances" cannot be relied upon 100%, but it helps to a considerable degree.

Temperature graph
Click for larger image


THE HEAT STORAGE POTENTIAL OF RADIANT HEATING SYSTEMS

It is important to understand that 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. A building that is not heated by a radiant floor will have a floor temperature that is lower than room temperature. A building that is radiantly heated by the floor will have a higher temperature.

 
high solar heat with cold air drain down

NON RADIANTLY HEATED FLOOR
  high solar heat

RADIANTLY HEATED FLOOR



YOU CAN STORE A LOT OF HEAT WITHIN THIS TEMPERATURE DIFFERENTIAL WITHOUT IMPACTING THE COMFORT CONDITIONS ABOVE.
  • It means that a radiant heating system with an intermittently available heat source (like solar) and a separate back up heating system (like forced air) can store a lot of heating energy without temperature and comfort excursions.

  • It means that under many conditions, you can simply dump a lot of solar heat into a radiantly heated floor without affecting comfort.

Then, if we accept a small night setback in temperature and a small difference between minimum acceptable temperatures and maximum acceptable temperatures, as in the passive manner, the storage potential of the floor becomes even greater. A temperature setback at night increases the storage potential of the floor further.




THE RESULTS OF DETAILED PERFORMANCE MONITORING


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

We predicted 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. The overall solar heating fraction was over 95% in the difficult Northern Vermont climate. It is important to understand that the project was not a real house; it was a residential scale test building without occupants. Nevertheless, the performance of this building and the performance of many subsequent occupied buildings suggest that the hybrid design approach enables high solar heating fractions. The entire Department of Energy report is available below.




US Dept of Energy Report
View entire report


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)



A paper presented at the 1985 meeting of the International Solar Energy Society
Click here

Performance Evaluation Of A Hybrid Solar Heating System Having A Radiant Panel Slab

This paper summarizes the results of performance monitoring of a simple hybrid solar heating system of the active charge/passive discharge type.

The Performance monitoring has demonstrated that this simple, low-cost hybrid solar heating system offers significant advantages in system efficiency, overall performance, comfort, and architectural flexibility.



New Shelter Magazine
View entire article


"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."

- From a ground breaking article in Rodale's New Shelter magazine.



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