sign in sign up
<<
Home , Ground source heat pump

Hybrid ground-source heat pumps SAVING ENERGY AND COST

www.ecw.org REPORT BRIEF

ybrid ground source environ­ mental­­­­­­ effectiveness of hy- Figure 2—THE ECONOMICS systems have the potential to brids in comparison to GSHPs and OF HYBRID SYSTEMS make conventional HVAC. H Invest in hybrid GSHP vs. conventional HVAC systems (GSHP) more cost effective. Though GSHP systems can significantly Our analysis found that all three instal- Fully invest in GSHP vs. hybrid reduce energy consumption in com- lations were economically cost effective. 14% mercial buildings, the high first cost of The average rate of return for investing installing the ground in hybrids in these three cases was 10%. 12% (GHX) can be a barrier. A hybrid system If they had invested in additional GHX uses conventional technology such as to go to a full GSHP system, the rate 10% a or (Figure 1) to of return on the additional investment TE OF RETURN (% ) meet a portion of the peak heating or would have averaged just 3% (Figure 2). 8% cooling load. This innovation allows you to install a smaller, less expensive GHX. Additionally, choosing a hybrid does not 6% sacrifice environmental benefits (includ- Comparing hybrids, GSHPs and ing carbon savings) because, in general, 4% conventional HVAC the supplemental equipment operates very 2% In a study sponsored by the U.S. De- infrequently due to the typical part-load ADJUSTED INVESTMENT RA partment of Energy, the Energy Center operation of these commercial buildings. 0 analyzed performance and economic Cashman East CTA Tobacco Lofts data from three hybrid installations to: Lessons learned • disseminate lessons learned, By monitoring and analyzing installed the design of these systems. Some of • validate models for others to use in hybrid ground source heat pump the lessons learned from study of these analyzing hybrid systems, and systems, the Energy Center was able buildings include: • assess the economic and to add to the body of knowledge on Component sizing in a hybrid system is Figure 1—A TYPICAL HYBRID SYSTEM extremely important—do not oversize

cooling tower the load that drives the GHX size. Use a sizing algorithm that optimizes the tow- er or boiler (see references and software heat pumps heat pumps options below; use hourly—8760— building loads as inputs if at all possible).

Pumping uses a lot of energy in a hybrid system. Minimize pump sizes and focus ground HX on part-load performance. For central pumping include a part-load pump of a smaller size (~50%). Consider using smaller individual pumps for “rogue” zones. Choose variable speed wherever applicable with a well-positioned dP sen- sor that is adjusted downward (post– occupancy) to allow for the lowest speed possible. REPORT BRIEF HYBRID GROUND-SOURCE HEAT PUMPS

CASE STUDY: EAST CTA 5–10ºF below the GHX. Condensing work very well in these systems.

Heat pump operation—use optimal or staged startup to avoid large peaks. En- sure proper return air paths and main- tenance accessibility to the units, and partially economize using outdoor air if possible.

FOR MORE INFORMATION Contact Scott Hackel at 608.238.8276 x129 or [email protected]

To read the full study, see: www.ecw.org/hybrid

Courtesy of SH Architecture Resources East Career and Technical Academy is a vocational high school in Las Vegas, Ne- vada. Individual, closet-installed heat pumps serve each space and are tied back to Software tools the via one large vari- HyGCHP—free version of the model FIRST COSTS able flow secondary loop. These sec- from this study, allows for comparison ondary building loops are connected of a variety of hybrid and conventional Cooling GHX Other to a primary loop, which has a 168,000 design approaches. tower cost cost costs ft GHX and two, multi-speed cooling EnergyPlus—free software from the U.S. $12 towers (167 tons each) attached. East Department of Energy for full building CTA is a heavily cooling-dominated modeling, including hybrid capability. building and investing in a full ground- $11 TRNSYS—full building thermal source system would have been cost- modeling, with the capability of studying prohibitive. By investing in the hybrid $10 any hybrid system imaginable. system, the district is saving approxi- mately $0.50/ft2 in operating costs an- Sizing tools—ground-source sizing tools $9 nually—only a few cents less than a full like GLD 2010, GCHPCalc and GLHEPro MILLIONS OF DOLLARS ground-source system. And the district have some very basic hybrid sizing $8 capability. GSHP Hybrid Conventional realized first cost savings of $1 million by going hybrid. Design references Basic design information: A Design Method for Hybrid Ground-Source Cooling towers or fluid coolers should speed. Precooling does not have to be Heat Pumps by Kavanaugh (ASHRAE be variable speed (if multiple towers, used to balance load on the ground. Transactions 1998); other work by ramp up/down together, not staged), Kavanaugh and Rafferty. and ramp down quickly enough to shut The loop should be able to bypass the Simulation and Optimal Control of Hybrid off shortly after substantial cooling. GHX. Set a reasonably wide deadband Ground Source Heat Pump Systems by Xu Tweaking the control setpoints after (20–30ºF) in which the GHX is not used; (Ph.D. Thesis, 2007); www.hvac.okstate. occupancy can ensure efficient operation. with GHX in cooling-dominated systems edu/theses.html coming on only ~10ºF below the setpoint If using nighttime precooling with a of the cooling tower. Optimal hybrid sizing; some sample cooling tower, operate it for a short pe- control sequences: http://sel.me.wisc.edu/publications- riod of time (a few hours), right before Boilers should be placed downstream theses.shtml (thesis by Hackel) morning startup, and at a lower of GHXs and controlled to a setpoint