Renewable Heating Design John Siegenthaler, P.E. | [email protected] Injection mixing from thermal storage An established technique that can be used in renewable energy systems. any hydronic-based renewable radiators on the second floor. The idea is to valves and are not familiar with variable- energy systems include thermal select a maximum supply water temperature speed injection mixing as an option. And M storage tanks. When the heat input for the entire system based on design-load while mixing valves can certainly accomplish is from solar thermal collectors or biomass conditions and then match the various heat the intended task, there are scenarios where boilers, it is possible for the water tempera- emitters to the room loads based on their variable-speed injection mixing may be less ture in those tanks to reach temperatures of respective heat output rates at the selected expensive because it provides both fluid transport 180-200º F. Thermal storage tanks supplied supply temperature. and mixing, thus eliminating the need for a by electric boilers operating on off-peak elec- The combination of high-temperature circulator in combination with a mixing valve. tricity also can reach these high temperatures water in thermal storage and low-tempera- by the end of the charging cycle. ture heat emitters necessitates some type of Hot in/Cool out Although few would argue a 500 gal. tank mixing assembly between these subsystems. Injection mixing is simply a way to add hot filled with 190º water doesn’t contain a lot of There are several mixing options includ- water to a circulating hydronic distribution sys- energy, the usefulness of that energy depends ing thermostatically actuated and motorized tem while at the same time removing an equal on the supply temperature required by the three-way mixing valves or injection mixing amount of cooler water. Replacing cooler water heat emitters to meet the building’s heating using either a motorized valve or variable- with hotter water means heat is added to the load. The lower this temperature, the greater speed injection pump. circuit. When heat is added to a circuit, its rate the temperature drop the tank can undergo The latter is a technique that, while of heat output increases. The faster hotter water and thus the more useful energy it can deliver. available for more than 25 years and applied is exchanged with cooler water, the greater the in many low-temperature systems supplied by rate of heat output. Aim low conventional boilers, has not found frequent With injection mixing, the two fluid In my opinion, all hydronic heating sys- use in systems supplied by renewable energy streams come together in a tee rather than tems, with or without renewable energy heat heat sources. Many engineers immediately within a valve. This fundamental concept is sources, should be designed to supply design associate the need for mixing with mixing shown in Figure 1. load output using a supply water temperature no higher than 120º. This upper threshold > Figure 1. consistently is attainable by most renewable heat sources such as solar collectors, air-to- distribution system water heat pumps, geothermal water-to-water load heat pumps and biomass boilers. It’s also complementary to systems using high-effi- ciency mod/con boilers. There are several heat emitter options that supply temperature sensor allow designers to assemble low-temperature purge heating distribution systems. They include complete mixing valve floor, wall and ceiling radiant panels, panel radiators, low-temperature fan coils, and even mixing begins here “low-temp” fin-tube baseboard. closely spaced tees Designers can combine multiple types of low-temperature heat emitters into the same system. One example would be a heated slab- cooler water out hot water in on-grade radiant panel on the first floor of a building with individually controlled panel Graphics courtesy of John Siegenthaler Note: The views expressed here are strictly those of the author and do not necessarily represent pme or BNP Media. 8 02.17 HYDRONICS KNOW HOW II > Figure 2. load BY JOHN SIEGENTHALER AND BNP MEDIA The Hydronics Know How II DVD contains all the col- umns and articles John Siegenthaler has written for purge Plumbing & Mechanical and PM Engineer, from the very fi rst in July 1996 through December 2008. That’s over valve flow restricting 200 articles and columns covering everything from heat loss to hydraulic separation. valve b/w tees This digital collection is a virtual encyclopedia on flow rate modern hydronic systems. No more cutting up magazines – This DVD makes it easy to access all of “siggy’s” articles indicator modulating valve and columns. Better yet, all the information is searchable. Just type in a key word or phase and Adobe Acrobat® will instantly search all the columns and articles for you. Good designers know that it never hurts to go back and review what you thought you completely understood a few years ago. Sometimes a small bit of new information can yield cooler water out major benefi ts when applied repeatedly in future systems. hot water in The information on this DVD will help you transform hundreds of individual components into smoothly operat- ing hydronic systems. Systems that consistently deliver load what their owners expect. In addition to the columns and articles, the Hydronics Know How II DVD contains the latest information on products, design and installation methods from several leading manufacturers. We are confi dent you’ll fi nd details that will further en- purge closely spaced tees hance the systems you design and install. If you don’t, we valve will refund you in full. provide hydraulic separation b/w distribution circulator SPONSORED BY: and injection pump PLATINUM GOLD BRONZE variable speed injection pump Any hardware assembly that can regu- Figure 3 (on page 10) shows a typical late the exchange of warmer water for cool- piping detail for injection mixing using a er water is a potential means of injection variable-speed pump along with formulas mixing. Common hardware arrangements that govern the rate of heat transfer. include a modulating two-way valve as well The variable-speed injection pump can be as with a variable-speed pump. These are located in either of the two injection “risers.” shown in Figure 2. It can push heated fluid into the distribution The temperature sensor that provides system or push cooler water from the distri- feedback on the supply water temperature bution system back to the heat-source circuit. + free to the distribution system should be located The latter keeps the pump slightly cooler. $ shipping downstream of the distribution circulator. The injection risers connect the heat- 99 This ensures complete mixing before the source circuit to the distribution system. The water passes by the sensor. closely spaced tees at these connections pro- TWO WAYS TO ORDER vide hydraulic separation between all three 1 // ONLINE at www.pmmag.com/products The math circulators. This is critically important since 2 // CALL Katie at 248/244.1275 The thermodynamics associated with the variable-speed injection pump is often injection mixing are simple and indepen- operating at a much lower speed than the Visit www.pmmag.com/products to dent of the hardware being used. Further- other two circulators. place your order or to view our entire more, injection mixing is fully “scalable” Hot water passes through the red injec- selection of technical resources. CALL KATIE @ 248/244.1275 WITH Graphics courtesy of John Siegenthaler from residential systems to large commercial tion riser at a rate determined by the speed ANY QUESTIONS. systems where heat transfer rates can be of the injection pump. Since the distribu- several million Btu/h. tion system is completely filled with fluid, pmengineer.com 9 PME AECStore_Hydronic Know How II DVD_Promo_0614.indd 1 5/16/16 3:30 PM Renewable Heating Design > Figure 3. to handle the injection flow rate (fi), rather closely from than the full distribution system flow rate spaced heat return (fd). If the heated water being injected is tees source to heat from the heat emitters, the significantlyinjection flow hotter rate canthan be the quite temperature small relative of to the distribution flow source rate. the water returning from the heat emitters, temperature = Tc temperature = Th from the heat emitters, thethe injection injection flow flow rate ratecan becan quite be quite small small relative to the distribution flow injection rate. relative to the distribution flow rate. risers variable For example: Assume water atFor 180 example: ºF is available Assume from water the atheat 180º source, is and that the speed available from the heat source and that the injection flow rate = fi distribution system serves radiant floor heating circuits that operate with a supply water injection sensor (S1)For example: Assume waterdistribution at 180 ºF system is available serves fromradiant the floor heat heat source,- and that the temperature of 100 ºF at design load conditions, with a corresponding return water temperature circulator temperature = Tmix ing circuits that operate with a supply water purge distribution system serves radiant floor heating circuits that operate with a supply water of 85 ºF. Also assume distribution system flow rate is 10 gpm. Under these conditions the rate valve temperature of 100º at design load conditions temperature of 100 ºF at design load conditions, with a corresponding return water temperature of heat dissipation by the distributionwith a corresponding system is: return water tempera- closely distribution of 85 ºF. Also assume distributionture of 85º. system Also flowassume rate distribution is 10 gpm. system Under these conditions the rate spaced circulator flow rate is 10 gpm. Under these conditions tees of heat dissipation by the distribution system is: the rate of heat dissipation by the distribution = − = heat systemQ 4 is95 shown(10)(1 0in0 Formula85) 71:4,250Btu / hr distribution flow rate = fd emitters Q = 495(10)(100 − 85) = 74,250Btu / hr = − The required injection flow rate to transfer heat to the distribution system at this rate can be 1 Q k( fi )(Th Tc ) found using formula 2 in figureThe 3.