Building solar into a radiant high mass floor

BY PETER BIONDO,

contributing writer

Solar is making a comeback! The interest is growing in the market now for solar space heating. One of the best green solutions for heating systems with solar thermal collectors is integration with radiant floor heating systems, particularly those with slab floors. The benefits of comfort, as well as energy savings, are well known for radiant floor heating, but I want to point out a less acknowledged advantage to heating a slab floor -- the opportunity to save 50 - 70% on operating costs with the integration of a solar thermal system.

Solar-integrated radiant floor slabs are about as practical a solution for a green heating alternative as you could imagine. With radiant floors, the building envelope can be integrated right into solar collection, because the slab floor is actively charged with the solar energy resource. The glycol-based, solar-heated water is cycled through a heat exchanger and into a storage tank and then circulated into the building. The architectural element of the building then lends itself very well to solar thermal transfer and distribution. The building envelope releases solar energy into the building by radiation and convection.

The distinct advantage that radiant heating has over other heat emitters using solar is that radiant slabs operate at low water temperatures, which are efficient for solar hot water collection. Lowering water temperatures minimizes collector losses to the ambient temperature, and harvests the solar resource with high efficiency. Water temperatures flowing through radiant floor tubing encased in a concrete slab would, depending on the outside temperature, operate between 80-120-degreeF. This is an efficient temperature range produced by medium temperature solar thermal collectors in most climates.

Solar radiant floor heating has grown in popularity due to meeting three primary criteria: cost effectiveness, system integration, and use in a wide range of climates. Solar radiant systems, to my knowledge, were first tested in the 1980s. The Department of Energy (DOE) contracted a report, “Technical Evaluation of a Solar Heating System Having Conventional Hydronic Solar Collectors and a Radiant Panel Slab,” done by the University of Massachusetts in 1983. The results of the report indicated that substantial improvements were observed in the system efficiency, overall performance, initial costs, and architectural flexibility over other types of active and passive solar heating systems. Research and development have shown that this solar application for heating offers advantages other solar space heating systems do not.

Radiant slabs operate at a relatively low water temperature, which increases efficiency and improves the overall performance of the solar collection system. As a result, fewer collectors are required, reducing the size of the system and initial cost.

Piping and controls for solar can be integrated into conventional radiant floor heating systems, either in new construction or retrofit. Collectors have some architectural flexibility for retrofit and can be integrated easily into new construction. The technology is applicable within a broad range of climate conditions where radiant floor heating systems are installed.

The thermal mass factor

What causes solar to do so well with a radiant floor slab is in the nature of a concrete mass for thermal storage and heat transfer. The benefit of a typical 4-inch concrete slab is its capacity for thermal storage capacity, particularly for solar energy.  At the same time, it is slow-acting enough to conduct and evenly distribute the heat at a comfortable temperature into the building. The properties of the thermal mass in concrete keep the collector’s working temperature running low and stable. The low operating temperatures with the slab floor will prevent wide temperature swings from occurring in the building. Indoor temperature swings can be further controlled using outdoor reset controls on solar storage or solar buffer tanks to the radiant loop.

With slab floors, the solar heating system can be piped directly from the collectors, through a mixing valve, and diverted into the radiant floor piping. No solar storage tank is necessary. If the slab is thickened to five or six inches, these systems do remarkably well. They fall in the lowest price categories and are usually designed for small buildings of 1,000-1,500 sq. ft. There are direct solar radiant systems for larger buildings as well. I know of two large solar companies that design direct solar radiant systems. These direct solar heating systems can have the solar antifreeze transfer directly into the floor tubing or through a flat plate heat exchanger, for antifreeze (solar) to water (radiant heating) separation. These systems can also be integrated for solar domestic hot water production for the off-load summer months. A 1,500-sq.-ft. house may require three to five collectors.

Normally, however, solar storage tanks are incorporated to integrate into a conventional heating system. These are called indirect systems. The solar storage tank is an important element of an indirect system. The solar tank adds two distinct features -- it separates the solar working fluid from the radiant distribution water with a heat exchanger, and controls the transfer of energy like a thermal flywheel. The solar tank is utilized in different ways during seasonal changes. In the winter, the solar tank operates more like a heat transfer station, exchanging low temperatures from the solar collector to the tank, from the tank, and into the floor. Usually loads are very high in the winter months and solar radiation is at its lowest, so the solar contribution cycles on and off throughout the day. In the fall and spring, with more solar radiation and milder days, the storage tank can collect more energy and additional temperatures. Higher temperatures stored in the solar storage tank each day are delivered mixed down to the radiant temperature required to avoid overheating the floor surface. In this case, there is more control over what hours the solar energy will be used to heat the building.

Solar thermal collectors

Solar radiant systems are installed with one of two basic collector models: flat plate or evacuated tube collectors. Solar thermal collectors are tested and approved in the U.S. by the Solar Rating and Certification Corporation (SRCC). Each collector model tested receives the SRCC certification “OG-100,” which is needed to qualify for commercial solar tax credits and rebates. The OG-100 rating for each collector model can be downloaded at www.solar-rating.org. 

The flat plate collector consists of an absorber surface, generally a sheet of copper or aluminum, coated with a thin layer of heat-absorbing material, referred to as a selective surface. Copper waterways attached directly to the absorber transfer the solar energy to the working fluid.  Insulation is placed behind the absorber surface. The assembly is contained in a “hot box,” an aluminum frame covered with a pane of low-emissivity tempered glass. The solar fluid is circulated through the collector waterways, drawing the heat out of the solar collector and into the storage system.

Evacuated tube collectors are comprised of evacuated glass tube heat pipes, each being a self-contained solar collector. A selective surface absorber is attached to a copper waterway running from the bottom to the top of the tube, all of which is sealed within a vacuum space free of air. The insulator for the collector is the vacuum space. At the top of the vacuum tube protrudes a condenser which is “plugged into” the evacuated tube collector manifold. The sun’s energy collected on the absorber heats the condenser tube and in turn heats the solar fluid, which passes through the manifold.

Both are good choices for radiant floor heating. Flat plate collectors work best in milder winter climates and climates with ample sunshine such as the southwest. Flat plate collectors are installed in every climate that produces the low temperature heat that is ideal for radiant floor systems. They are widely available and come in sizes from 3 x 8 ft. to 4 x 10 ft.

Evacuated tube collectors, on the other hand, produce a greater fraction of solar energy for heat transfer in colder winter climates and those with cloudy conditions. Collecting solar energy in a round tube within a vacuum seal increases the collection of scattered solar radiation during cloud cover. The vacuum space, free of air, is an excellent insulator against heat loss and, as a result, more solar energy is delivered as cold winter daytime temperatures work against warm or hot solar fluid temperatures. Evacuated tubes collect especially well over a flat plate collector when cool winds are taken into account. They also generate high temperatures more efficiently, which increases the thermal storage capacity of a solar storage tank. This form of technology to collect solar energy is more expensive than flat plate collectors and, in snow country, should be sloped at a tilt angle of 50-degree or more to shed snow.

The results from performance tests conducted by Solartechnik or SPF, a European agency engaged in applied research and development on thermal solar technology since 1981, indicates that vacuum tube collectors can produce twice the energy over flat plates in respect to aperture area of the collector. The design criteria used in their performance test is for the climate in central Switzerland, tilt angle of the collectors at 45-degree facing south, and a solar system sized to provide 25% of the annual heating load.

Establishing an installation location for the collectors that will not be shaded by trees, chimneys, or other buildings is critical. Orientation and tilt inclination are also extremely important. Special consideration to tilt angle is necessary for space heating systems. The collectors must be able to capture the low sun angles during the winter months. The proper tilt angle for the collectors is a region’s latitude angle plus 15-degree. In Denver, for example, the proper tilt angle is 40-degree latitude plus 15-degree, which is equal to a collector tilt angle of 55-degree. The collectors must also be oriented within 20-degree of true south. There are isogonic charts that map out magnetic variations for true north from magnetic north, so the necessary changes can be made from the magnetic reading on the compass.

These collector installation criteria will help you in the preliminary assessment to determine whether the solar collectors may be integrated for roof or ground mounting. The best option is to design the building to include solar collectors that take advantage of solar energy.

A brief word on collector size and system cost

Solar space heating systems have a much larger collector area to meet a space heating load than the typical solar domestic hot water heating system.  Most of us have seen solar domestic hot water systems usually with one or two collectors tied to an 80-gallon storage tank. I have spoken with many people who believe that if they add another collector, they can do space heating. That would be true if they lived in a 300-sq.-ft. home! The size of the collector area depends on: 1) the heat loss characteristics of the building; 2) the winter climate; and 3) the solar radiation available.

For example, let’s take a 2,400-sq.-ft. building. In the northern climates, the collector area may require 200 - 250 sq. ft. of solar collectors for a 50% solar savings. In the Sunbelt, the mountain states, the southwest, and the mid-Atlantic region, that same building may need only 120 - 200 sq. ft. of solar collectors for a 50 - 60% solar savings.

What determines cost is the installer, the type of collector installed, storage tank choices, and the price of labor. This type of work can often fall into the hands of inexperienced contractors willing to learn as they go, and the design and the cost of the installation reflects that. Using a licensed and insured mechanical contractor who can ensure clean installation with quality materials and efficient solar collection and distribution, one can expect the installed system cost to range between $15 – 20 per sq. ft. of space-heated area, for an indirect solar retrofit into an existing radiant floor system.

With the rising costs of fuel, solar is being given another chance to shine. Return on solar investment is an interesting thing to study, especially with the wide range of costs that fossil fuels have per unit of Btu energy. For example, consider a propane or fuel oil heated radiant panel system tied to solar. That solar investment will pay for itself three times faster than the same system using a natural gas boiler. I know successful contractors who target fuel oil and propane users primarily, because they can sell these systems on the return of the solar investment in seven to ten years or less! And who knows where the rising price of fossil fuels will taper off. Solar is not only an efficient match for radiant floor heated slabs, but is also a great way to leverage an investment against rising utility rates and the high costs of fuel oil and propane.                   

Peter Biondo is technical sales coordinator and head of design for Oventrop Commercial Solar Division. Peter has been involved in the solar heating industry more than 20 years and worked on over 3,500 solar heating systems nationwide. He carries a mechanical contractor’s license in the state of Arizona. He can be reached at peter.biondo@oventrop.com.