Stranded heat recovery using software controls
By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.
Over the past 10 years or so, I have got into the habit of designing hydronic heating systems by linking all my heating sources to all my heating loads using the principles of primary loop piping to create a “flow center” where everything is connected. This can be accomplished with site-assembled primary-secondary (P/S) components or duplicated using hydraulic flow separators and pump modules pre-fabricated “off the shelf” from manufacturers like Caleffi, PHP or Taco.
In these articles I have been focusing on piping diagrams showing P/S piping, because this is a generic way to install solar hydronic combisystems where the flow paths are relatively easy to visualize.
Figure 30-1 shows a typical piping configuration, similar to dozens of combi-solar home heating systems that we have installed and commissioned in recent years. We have found that, when the piping is always done in a standard way, the controls can be standardized as well. That way, many different installations can be controlled using the same configurations, making the final result more familiar and reliable. This allows the installers to avoid the perplexing task of acquiring custom control equipment for every job and then learning how to adjust it properly “from scratch” every time. It is better when experienced installers can focus on using standardized and familiar controls for fine-tuning to optimize the energy efficiency of each installation.
For the past two years at SolarLogic we have been developing centralized controls for solar heating combi-systems that allow the standardization and tuning for energy efficiency to advance to a whole new level. We do this with a control system that is based entirely on software and is completely interactive by remote control over the Internet. We can watch the performance of all the heating system components from day to day (or minute to minute, if needed) and “tweak” the control settings any time we see an opportunity for energy improvement. And we can do all this from the comfort and convenience of our office computer or any Internet computer that happens to be nearby. We call this system the Solar Logic Integrated Control (SLIC).
Case Study: Before and after improved control strategy
Figure 30-2 shows an example of an immediate improvement in heating efficiency accomplished with a simple time delay. The data in Figure 30-2 was recorded by our SLIC control system and used to verify the success of this energy efficiency strategy. Here is the situation. When a “hot water baseboard” zone calls for heat, the boiler will typically fire to provide the high temperature fluid required by these fin tubes.
The diagram in Figure 30-1 shows the actual piping configuration. The boiler fluid can be preheated, either by the solar collectors directly through the heat exchanger or by the solar heat storage tank. The boiler produces the final temperature required for the baseboards, 175 F. The zone (a small bathroom) heats up quickly, and the room thermostat shuts off the call for heat. In a typical heating system, the boiler flame would stop, the boiler pump would stop and, if the solar collectors were cold, the solar pumps would stop. At this point 175 F boiler fluid is stranded in the primary loop. This situation can occur multiple times per day during the heating season.
The top graph in Figure 30-2 shows how this stranded heat cools off over a period of hours. The wasted heat goes into overheating the boiler room; for reference, you can see the Domestic Hot Water (DHW) tank sitting there at 125 F, also losing heat very slowly to the boiler room. When we noticed this situation, we wanted to stop heating the boiler room and use the stranded heat for some practical purpose.
We changed the settings in the SLIC so that the DHW pump and the primary pump continued to run after the boiler flame shut down. Since this change required only software instructions, no hardware changes were required. The instructions included some other details like turning off the heat recovery pumps when the temperatures are reasonably low and not trying to do heat recovery when the normal heating functions are operating. You can see in the bottom graph in Figure 30-2 that it takes less than 10 minutes to recover all the useful heat from the primary loop and deliver it to the DHW tank. The temperature rise in the DHW tank is unmistakable, driven entirely by “waste” heat.
Hardware versus software
I think it was Amory Lovins who once compared the pursuit of energy efficiency to dining on a whole lobster. About half the reward is easy to find and comes in large pieces (the tail), but the other half is in smaller pieces and takes more skill and attention to obtain (from the claws, legs and other bits). In our Solar Combisystems, the big rewards are in the “free” energy delivered by the solar collectors and the fuel savings of a condensing backup boiler (if included). But the smaller opportunities for ever-increasing energy efficiency are numerous, accomplished simply by moving heat from one place to another at the right time with intelligent control.
If we were to try to implement this kind of precise control using conventional hardware, it would require (at least) a time delay relay and perhaps a set-point or differential thermostat and some other interlock switching to produce the same intelligent heat recovery shown in Figure 30-2. The hardware would require a site visit, skilled electrical installation and some testing to complete. Our software approach was implemented, installed and verified remotely and required no additional hardware.
The amount of heat recovered may seem small, but if this heating condition occurs several times a day throughout the heating season, the savings will be equivalent to several gallons of heating fuel per month for the rest of the life of the system. This kind of savings justifies the effort to change the software but might not justify the cost of the site visit and hardware installation when doing it the old fashioned way.
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. Special thanks to Dr. Fred Milder at SolarLogic for providing the original graphics and data included in this article.
Bristol Stickney, partner and technical director at Cedar Mountain Solar Systems in Santa Fe, N.M., has been designing, manufacturing, engineering, 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 and is involved in training programs for solar heating professionals (visit www.cedarmountainsolar.com or www.solarlogicllc.com for more information.)