Solar Heating Systems: Storage mediums and temperature control strategies

for predictable and consistent performance

Residential solar installations are on the rise nationwide, but deployment of solar space-heating systems lags well behind photovoltaic, solar pool heating and domestic water heating technologies. A variety of factors contribute to this trend. Foremost among them is the complexity of space-heating system design and installation, especially heat storage and system controls. Space heating represents more than 40% of home energy use. As more proven designs and modular components are brought to market, many heating contractors will routinely offer solar heating.

For solar heating to become mainstream, systems need to have consistent, predictable performance and offer a user interface that is comparable to conventional heating systems. The customer should have to adjust only the thermostat, and the rest of the system should take care of itself. If the control approach sheds excess solar heat into a concrete floor, the floor’s temperature must not fluctuate dramatically. Customers do not want rooms overheating due to excessive system output. If a hot tub or swimming pool is used to shed excess heat, a safe highlimit temperature must be maintained.

To ensure quality solar heating installations, you must carefully balance customer satisfaction with the proper operating parameters of the solar collectors. A key recurring challenge in closed-loop hydronic solar heating design and implementation is the need to manage excess solar heat, especially in the shoulder seasons—spring and fall. One method to manage excess solar heat is by the use of water storage tanks as heat sinks. However, this involves added system costs, space requirements and complexity as well as the need for control strategies when the water in the storage tanks gets too hot.

At Cedar Mountain Solar, we developed control strategies for closed-loop glycol systems that facilitate automatic solar heat storage in the thermal mass of buildings while mitigating potential overheating issues. The methods we use are appropriate for the high desert climate in New Mexico and locations with similar characteristics, which include wide diurnal temperature swings, cold winters and ample sunlight.

WATER AND CONCRETE AS HEAT-STORAGE MEDIUMS

Compared to concrete, water has a higher heat-storage capacity but a lower density. In solar heating systems, concrete is often available in a much greater volume than water. These variables complicate the comparison between the two heat-storage approaches. To illustrate the difference in performance of direct solar-heated concrete to the more common solar-heated water tanks, we can employ a simplified analysis of two hypothetical heating systems modeled for the climate in New Mexico. We round off the numbers and make assumptions based on our experience to get into the ballpark for a reasonable comparison.

A good snapshot of these two heat-storage systems must include storage capacity as well as heat loss from the different configurations. The specific heat capacity and the density of the heat-storage material define the storage capacity. The heat loss is driven by the temperature difference between the warm mass material and the environment, the insulating value and the surface area. Table 1 summarizes the key conditions needed to make a comparison. We calculate heat loss by multiplying the surface area by the temperature difference and dividing by the insulation’s R-value. We then calculate heat storage by multiplying the specific heat by the density and then by the temperature rise (or drop) in the material.

A HYPOTHETICAL SOLAR-HEATED HOUSE

Consider an energy-efficient residence with 3,200 square feet of heated living space with the energy use and performance temperatures shown in Table 2. The owners decide to integrate eight 4-by-10-foot flat plate collectors to supplement a hot water heating system that uses a hydronic boiler. One option is to store the heat produced by the collector array in water tanks. The second option is direct heat storage using insulated, slab-on-grade hydronic radiant concrete floors. The size of the collector array is typical of systems installed in the area and represents about 10% of the floor surface area. Water-tank storage for this system is typically sized to provide two gallons for each square foot of collector area, or 640 gallons. Setting aside the other obvious design issues such as integrated DHW, room temperature-control strategies and protection from overheating, we can focus on how much heat generation is involved and how the thermal storage systems react to it.

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