#49: Case Study: Solar combi 101 – summer data snapshot
By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.
The typical solar-hydronic combisystem in our region is composed of (1) a bank of flat-plate solar heat collectors, (2) a gas boiler backup, (3) a domestic water heater (DHW) and (4) a radiant heated floor. I call this popular application “Combi 101” and typically connect the components together in a standard way using a primary loop piping configuration. In late 2011, a system just like this was installed as a retrofit at a home in Canoncito, N.M. The installed solar combisystem includes all the important and innovative features mentioned in previous articles, including:
• Radiant masonry floor winter heating and summer cooling using flat-plate solar panels;
• DHW heating in both winter and summer;
• Room-by-room, two-stage temperature control using distributed heat storage in the mass floors;
• Solar DHW and tank cooling (tank overheat prevention);
• DHW recirculator control by time, temperature or demand switch;
• Solar collectors with self-cooling thermosyphon fins.
The same glazed flat-plate solar panels are used for heating and cooling applied to both the space heat and the DHW tank. Heating or cooling provided by the panels is distributed to the masonry radiant floors and the DHW tank using a hydronic primary loop piping configuration. A single backup hot water boiler provides conventional heat to both the floors and the DHW automatically whenever needed.
This solar home heating system has been outfitted with a SolarLogic integrated controller (SLIC), which provides all the control functions and connections in a single control box. The SLIC control system includes the capability for continuous data recording as a standard feature, which can be observed and downloaded via the Internet. This permits me to download fresh data from the field on demand, to illustrate the performance of this well-behaved solar combisystem. The data presented here is from two hot summer days just prior to the 4th of July, when both hot water and space cooling are desired and reliable overheat protection is imperative.
Case Study: Summary and description
The single-story house has just over 2,000 square feet of heated space and is well-constructed, with high ceilings in the center. It has passive solar features, including interior masonry walls and floors and roof overhangs providing summer shading to help moderate the temperature swings in the building. The original hydronic heating system included four heating zones with separate room thermostats and four zone valves feeding a total of 1,250 square feet of masonry radiant floors. The only heating fuel available is propane, and a Knight WBN106 hot water boiler with an AFUE rating of 95.5 percent provides the backup heat. The DHW tank is an 80-gallon Triangle Tube indirect water heater tank that can be heated by solar or boiler using its tank-in-a-tank heat exchanger.
The collectors are the SS16 model made by Solar Skies, as seen in Figure 49-1. A total of 256 square feet (gross) of collectors is used by the control system to automatically provide solar heating by day and night sky radiant cooling (NSRC) by night.
The climate at this location is that of a high-altitude arid mountain region at over 6,800 feet elevation. It can be extremely sunny and dry for months at a time, but in winter there is often snow, and when it rains there can be flash floods.
Nighttime temperatures during clear weather can be 30 to 40 F colder than the daytime. The original house and the heating system retrofit were designed to take energy advantage of these climate conditions.
The control system is designed to allow DHW to be solar heated by day, while the masonry floors can be cooled at night. This is a standard feature built into every SLIC control system. When no heat is needed in the afternoon, the collector piping and controls allow for self-cooling.
I would like to remind the reader that this is not an experimental solar heating system, but rather a consumer installation similar to dozens of installations deployed over the past five years. The design, installation and control methods used here can be quickly and easily duplicated in retrofits and new construction in a wide variety of climates.
Data snapshot: highlights of summer performance
The weather was very hot during the two days of this case study period, with some occasional scattered clouds and light rain, which can be seen to cause the outside temperatures to rise or fall suddenly. Figure 49-2 shows the temperatures recorded between July 1 and July 3, 2012. The control functions can be seen as temperature responses on the data graphs, which I have labeled for easier identification. Following is a brief explanation of the data highlights.
• NSRC cooling on/off: The space cooling system turns on and off in response to the availability of cold night radiant temperatures and calls for cooling from room thermostats. On the first night, the cooling system circulates all night long and provides a final room temperature of 73 F. On the second night, it shuts off early in the morning when the target temperature of 72 F is achieved in the rooms. The homeowner has set the room thermostats to cool only two of the four possible zones, thereby concentrating the night cooling in the most important rooms. This is done by setting the “solar” switch on the SLIC room thermostat.
• Manifold A — fluid from the floor: At night, the warm fluid from the floor can be seen passing through Manifold A. When compared to the cold fluid available for cooling seen as the solar input temperature, we can verify that the NSRC fluid is indeed cold enough to help chill the floors. Heat is drawn out of the masonry floors overnight while the fluid temperature in Manifold A drops steadily from a starting point of over 76 F to below 70 F by early morning.
• Outdoor temperatures: Ambient and white plate sensors are in a very hot location above a metal roof. The ambient (outdoor air) sensor tends to read higher than the official weather report. During this test period, we are scorching on the roof by day, with sensor temperatures hovering around 120 F while a nearby weather station reports a high of 96 F. At night, the white plate indicates that the coldest possible NSRC temperatures are in the 50s and 60s F. The plate sensor is used by the SLIC system to signal when potential cooling is available to activate it. During the day, the white plate shows a crude indication of how sunny it is when the sun heats it well above the air temperature.
• DHW recirculation on/off: This function is built into the SLIC control system so that energy consumed by recirculation can be monitored and controlled. The homeowners have this set to run only at certain times of day. When it is running, there is an obvious rapid drop in temperature at the top of the DHW tank.
• Solar panel self-cooling: The control system constantly delivers heat from the solar panels to the boiler room whenever available during the day. When there is no demand for heat, the solar circulation pumps turn off and thermosyphon cooling fins keep the collectors from making steam. This passive cooling system continues to work normally even during a power failure. When self-cooling, the data shows the solar panel temperature to be hovering above and below the 200 F mark.
• DHW tank heating/cooling: Each day, the temperature in the DHW tank can be seen to rise from 120 F to the operating high limit of 170 F, using only solar heat. The control system is capable of shedding heat from the DHW tank when the water in the tank reaches a high temperature limit. This heat dissipation function is adjustable, and in this installation it is currently set to allow the tank to absorb heat up to 170 F. It then cools it to about 155 F after sunset, using the NSRC cooling function, but at a much higher temperature than space cooling. At these high temperatures, the tank can be cooled very quickly by pumping the solar collectors “in reverse.” The controls are set to provide plenty of solar hot water at reasonable temperatures while using little or no backup heat from the boiler in summer.
Room temperature results
The whole point of NSRC is to enhance comfort cooling in the indoor rooms during warm weather. This subtle effect can be seen in the room temperature data as it drifts upwards by day and is pulled down slowly overnight. The room temperatures fluctuate between 78 and 70 F during this test period. There are no A/C or evaporative coolers in this building. And, in case you are wondering, the solar collectors did successfully provide heat for the building during the past cold winter. But that is another story.
All of the components and control concepts presented above have been defined and discussed in greater detail in earlier episodes of this series of articles. Archives and links to past articles can be found on the websites of TMB Publishing and SolarLogic LLC.
These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on pressurized glycol/hydronic systems since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement.
Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.