The Need for Thermal Storage in Solar Heating Systems
Any type of solar heating system, whether it be active solar or passive solar heating, needs thermal storage, because there is no sunshine at night and limited solar energy available on cloudy days. As a result, most solar heating systems are designed to collect more solar energy during a sunny day, than is needed for heating the home. The extra daytime solar energy is stored for use at night and on cloudy days. A system of heat storage for solar hot air systems using concrete blocks was patented in 1978, but hasn’t really become a mainstream method for solar heat storage. Read on for information about typical current heat storage methods for active solar and passive solar heating systems, and their comparison with heat storage using concrete blocks.
A 1978 Patent for Solar Heat Storage Using Concrete Blocks
On December 5, 1978, U.S. patent #4127973 was issued for a "Solar-Heated Concrete Slab Building Structure." The basic idea presented in this patent is the use of a layer of hollow concrete blocks below a concrete slab floor, with the hollow concrete blocks lined up so that air can flow through from one side of the floor to the other, as shown in the diagram at the left. In this design, the concrete slab floor provides most of the thermal mass (thermal heat storage capacity). The concrete blocks serve as a plenum below the floor to circulate heated air. The air could move by natural convection or a fan or blower could be used. The first reference at the end of this article provides more details about this patent.
Properties of Solar Thermal Storage Materials
A good solar thermal storage material should have a high capacity to store heat per unit volume. It should also absorb and release
heat at a reasonable rate (not too fast and not too slow). The properties to be compared for various candidate solar thermal storage materials are thus the heat capacity (specific heat) and the thermal conductivity. The table at the right was prepared to show a comparison among some materials used for solar thermal storage and some that wouldn’t work well for solar thermal storage. The specific heat of a material is a measure of the heat storage capability per unit mass per unit temperature difference. Since we are more interested in the heat capacity per unit volume than per unit mass, specific heat was multiplied by the density to get the volumetric heat capacity shown in the table. The thermal conductivity of a material is a measure of the rate at which heat will pass through the material per unit area per unit of temperature gradient.
The first group of materials in the table includes some used in home construction and some potential solar thermal storage materials. Notice that water has the greatest capacity to store thermal energy (heat) per cubic foot. In fact, a cubic foot of water holds nearly twice the amount of heat as most of the other materials in the list for each degree of temperature rise. Also notice that most of the materials in the first group have a thermal conductivity in the range from about 0.1 to 1 Btu/hr-ft-oF, which is a ‘reasonable rate’ of heat absorption and release. By comparison the metals at the bottom of the list have a much higher thermal conductivity. They would heat up very fast, but would give off their stored heat too fast to be useful as a heat storage material. The insulating materials shown have much lower thermal conductivity. They would heat up too slowly to be useful for solar thermal storage.
See page 2 of this article for information about current typical solar thermal storage materials for active and passive solar heating systems and their comparison with the use of concrete blocks for heat storage.
Thermal Storage in Active Solar Central Home Heating Systems
Active solar central home heating systems may use either air or water as the working fluid. The diagram at the left shows a solar air heating system. Typical practice with an air heating active solar system is use of a bed of rocks, as shown in the diagram, for storage of excess collected daytime solar energy by heating up the rocks. Drawing air through the bed of hot rocks at night time provides warm air for heat. As shown in the table on the previous page, a bed of rocks has a reasonable heat storage capacity and an appropriate thermal conductivity value.
A water heating, active solar heating system heats water in the solar collectors and circulates hot water, rather than air. The heat storage for a hot water solar heating system is quite simple. All that is needed is an appropriately sized insulated tank of water. The water is heated with excess collected solar energy during the daytime, and is used to provide night time heat. As shown in the table on the previous page, water is very effective for thermal storage. It has a high heat capacity and an appropriate thermal conductivity value.
For more background about active solar central home heating systems, see the article, "Central Heat from Solar Energy?"
Thermal Storage in Passive Solar Heating Systems
The essential components of a passive solar heating system are adequate south-facing windows (in the northern hemisphere – or north-facing in the southern hemisphere) to allow solar radiation to shine into the house, and adequate thermal mass to absorb
incoming solar energy during the day time (as shown in the diagram at the left) and release it to provide heat at night (as shown in the diagram at the right). The thermal mass for passive solar heating systems typically consists of masonry or concrete floors and walls.
As shown in the table on the previous page, concrete, brick and adobe all have similar, reasonably high volumetric heat capacities and appropriate thermal conductivity values.
For more background about passive solar heating systems, see the article, "Principles of Passive Solar Heating Systems and How They Work."
Ok, now you’ve seen information about a patented system for using concrete blocks in a solar heating system, some material properties information, and information about materials used for thermal storage in active and passive solar heating system. Is heat storage for solar hot air systems using concrete blocks an up and coming thing?
Here’s my opinion: While the use of hollow concrete blocks to create a path for air flow past a heated surface might have some use, concrete blocks would not work as well for solar thermal storage as the materials currently in use. For active solar heating systems, excess solar heat couldn’t be transferred to the concrete blocks as efficiently as it can be transferred to either a tank of water or a bed of rocks. For a passive solar heating system, a solid concrete or masonry wall or floor is better because it will store more heat per cubic foot than hollow concrete blocks.
References and Image Credits
References for Further Information:
1. Solar-Heated Concrete Slab Building Structure – U.S. Patent #4127973
2. New Mexico Solar Energy Association, Passive Solar Design Guidelines for Northern New Mexico
3. Arizona Solar Center, Passive Solar Heating and Cooling Manual
1. Concrete Block Diagram – drawn by H. Bengtson based on information in U.S. Patent #4127973
2. Solar Central Home Heating System – Bengtson, H., Solar Energy Fundamentals, an online, continuing education course for PDH credit.
3. Passive Solar Heating/Direct Gain Drawings – Bengtson, H., Passive Solar Heating Basics, an online, continuing education course for PDH credit.