The Los Alamos Eco Station is the headquarters for the recycling center in Los Alamos, N.M., originally built in 2008. We have featured it in two earlier articles because of its award-winning renewable-energy design. It is a small commercial office building of approximately 1800 square feet with a commonplace appearance. But, with a LEED Silver rating, solar hydronic combisystem, night radiant cooling, continuous data logging, remote monitoring and remote control, it provides a window into cost-effective energy-efficient building performance that may be unique in the world.
The building uses a “New Standard” hydronic primary loop heating system that ties the solar heat collectors together with the domestic hot water, radiant heated floors and a backup boiler all under central control. There is a separate bank of flat plate panels that provide cooling in summer. Thanks to a SolarLogic Integrated Control (SLIC) system the solar space heating, night cooling, solar DHW and backup boiler are all interconnected and fully functional without the need for any large heat-storage water tank.
The system eliminates the need for a large heat storage tank by using the heat storage capacity of the concrete radiant heated floors themselves under intelligent control. Since the SLIC control system (upgraded in 2012) contains an integrated data logger and energy metering, we can now look back over the past year to see how the renewable energy system has performed.
How does the solar heating system work?
The solar heat collectors can be seen on this building in the photo in Figure 70-1 flush-mounted on a vertical wall facing south. There are 10 flat-plate panels connected together as one bank, for a total of 251 square feet of net collector area. These are glazed, well-insulated Vitosol 100 collectors from Viessmann. The vertical tilt allows the collectors to provide their maximum heat delivery at the low sun angles in mid-winter when it is needed most for space heating. In summer, the high sun angle provides just enough solar heat for domestic hot water in the building at this collector tilt which naturally helps to prevent overheating in summer.
In winter, the primary-loop piping configuration allows solar heat to be delivered directly to the concrete radiant heated floors, bypassing the domestic hot water tank. It can also deliver heat to the water tank while bypassing the floors or it can do both heating jobs simultaneously. In this type of standard plumbing configuration, the heat delivery is controlled simply by turning hydronic circulator pumps on or off. The heating decisions are made by the SLIC controller based on the temperatures available and the priority settings for the heating jobs that are calling for heat. Two stage room thermostats are used to establish priority in the different space heating zones.
Room thermostats: Two stage solar heat control
When a room thermostat sends a “stage one” call for heat, the controller waits until solar heat is available. By day, when the controller senses that the collectors are hot enough to do the job, the solar circulator is turned on along with the corresponding zone valve and floor circulator for the room calling for heat. Solar heat is delivered directly into the concrete radiant floor, and the thermal heat storage capacity of the concrete begins to absorb and store the solar heat. Over time, the thermal mass of the floor warms up and a great deal of solar heat is then stored directly in the concrete in the room ,at a relatively low comfort-temperature. When the surface of the floor begins to provide the room with a high enough temperature, the upper limit of the “stage one” set-point is reached, and the delivery of heat is stopped to prevent that room from exceeding the range of human comfort. If more solar heat is available, the controller sends it to the other rooms that have not yet reached their high-limit temperature, or the heat is sent to the hot water tank. In this way, the controller distributes solar heat throughout the building, storing it in the thermal mass of the floors directly, as well as the DHW tank.
If the room temperature drops below the stage one setting, a “stage two” call for heat occurs. This tells the controller that the solar heat is not keeping up with the heat-loss in the rooms, and the backup boiler is allowed to contribute. The controller allows the boiler circulator pump to turn on, along with the zone valve and circulator pump associated with the room thermostat that is calling for heat. The controller will allow the solar heat to continue circulating only if it is hot enough to provide pre-heat for the boiler. The controller may cycle the solar contribution on and off during a partly cloudy day to control the pre-heat temperature. The room thermostats are adjustable by the occupants, so they can choose a target temperature that is comfortable without even being aware of the two-stage control logic going on in the controller.
The solar heating performance in 2013
The energy graph seen in Figure 70-2 shows solar heat energy delivered to the building month by month, for 2013. This heat is measured in the primary loop as it is being delivered to either the radiant floors or the DHW tank. So, it is not just a theoretical solar heat availability, but it is the actual solar heat delivered to the building. Since it was consumed for a useful purpose in the building, it represents actual fuel offset that would have been needed from the boiler. The actual fuel savings is a little higher than seen in this graph, since the graph does not include the wasted fuel that would have gone up the flue pipe if this heat had been provided by the boiler. For this condensing boiler, the flue loss is small, on the order of 5 to 7 percent.
Using an energy graph like this, the building owner or operator can quickly verify that the solar heating equipment is functioning in a reasonable and reliable way. For example, this graph shows that the collectors are putting out plenty of heat in winter, which compares well with the amount seen in the SRCC test ratings for these collectors. And, it proves that there is just enough solar heat to provide the minimal summer DHW heat load, showing the effectiveness of vertical collectors in this installation.
Using other data recorded by the SLIC control system, the electrical energy consumed by the circulator pumps can be determined. When you compare the heat-energy delivered, to the pump-energy consumed, you can calculate and equivalent Coefficient of Performance (COP), which is an interesting way to compare the efficiency of the solar heating system. In this case, on a typical day in December, if the solar pumping system runs for 5 hours per day, and delivers 167,000 BTUs/Day, the COP can be estimated at just over 47. In other words, for every unit of energy it takes to run the circulators for solar heat, 47 units of useful heat energy are delivered to the building. This is a very high-performing “heat pump” indeed.
How does the NSRC Cooling System work?
The Night Sky Radiant Cooling (NSRC) panels can be seen on this building in the photo in Figure 70-3, flush-mounted on a nearly-flat roof. There are 12 flat-plate panels connected together as one bank, for a total of 576 square feet of surface area. These are unglazed, copper fin-tube panels from Sun Ray Solar (4X12). The horizontal tilt allows the panels to have a clear view of the night sky, so that heat may radiate from the surface of these metal plates to the cold night sky in a direct and unobstructed straight line. On a clear night in summer, the metal plates will lose heat so rapidly by radiation to the cold sky that the surface of the panels will drop well below the surrounding air temperature. These cold panels are used as a source of cooling for the hydronic fluid in the building, and cool fluid can be pumped through the concrete floors, typically all night long in summer, to pre-cool the concrete floors in advance of the next warm day.
The SLIC control system is switched to Summer Cooling mode, and that allows DHW solar water heating by day, and NSRC radiant floor cooling by night. The summer night operation is very similar to the way a solar heat collector is controlled, except all the thermal control decisions are done in reverse. So, when a room thermostat is set for night cooling, the system waits until the cooling panels on the roof are cool enough to do the job. The circulator pumps turn on to move glycol fluid through the cooling panels, which is used to chill the fluid being pumped through the floors in the rooms calling for cooling. The same heat exchanger is used for cooling at night that is used for solar heating by day. When the concrete radiant floors are pre-chilled at night, the rooms stay cooler than normal well into the next warm day, which reduces (or eliminates) the run-time of the conventional air conditioner (A/C) in the building the next day. Since the chilled floors are controlled by the zone valves and room thermostats, they will turn off the cooling when a comfort-temperature has been reached in that zone.
The NSRC (Cooling) Performance in 2013
The energy graph seen in Figure 70-4 shows NSRC (cooling) energy delivered to the building monthly for the year 2013. This cooling energy is measured in the primary loop as heat is being removed from the radiant floors. So, this is the actual heat removed from the building by the NSRC system measured with a BTU meter. Since this energy was removed from the thermal mass of the floor, inside the insulated envelope of the building, it represents an actual shift in the comfort temperature in the building that will offset the run-time of the conventional A/C, which operates on its own separate thermostat and control system.
Figure 70-4 shows that the cooling system typically delivered 50 – 60,000 BTUs per night throughout the cooling season in 2013. When we looked up the data for the pumping energy required, the COP was estimated in the range of 7 to 18. The NSRC control system was modified and adjusted a number of times during 2013, so the cooling performance has not really been optimized yet. But with this data, we can at least verify that it was working consistently during summer 2013 and with a COP (efficiency) that is much better than your average air conditioner. This system was installed with conventional circulators that use twice as much electricity as newer ones. If state-of-the-art circulators are installed to replace the old ones, the COP ratings could easily double.
The information presented here is just a quick reality-check based on the energy data that is easy to see on-demand using the SLIC dashboard style display. Of course, the data exists in the log files on this system to inspect and evaluate anything of interest in much more thorough detail whenever needed.
These articles are targeted toward residential and small commercial buildings smaller than ten thousand 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.