What does the kind of lighting you select for your building have to do with the cost of your ground heat exchanger (GHX)? In residential homes not so much, but in most commercial or institutional buildings it can have a lot to do with it. Lighting has a large impact on the cooling requirements for a building, especially a building that is occupied all day. Many standard lighting system designs draw 1.5 to 2.5 Watts per square foot (16 to 27 Watts / m2) of occupied space. If the lights are turned on for ten hours per day, the lighting in a 50,000 square foot (4,650 m2) building will add approximately kW of heat to the building.
In winter that energy helps heat the building, but in summer, at 2 Watts per square foot (21.5 Watts / m2) that's an additional 100 kW (341,200 Btu/hr, or over 28 tons) of cooling that the system has to deal with. More efficient lighting can reduce that by 40% to 60%. That's heat that the GCHP system doesn't have to supply in winter, and cooling the system rejects to the ground the rest of the year.
The interesting thing about a GHX is that when the amount of energy rejected to it is approximately equal to the amount of energy extracted from it over the year, the size of the GHX can reduced. Remember, a GHX is not an infinite energy source...it's much more a storage medium.
If the heating and cooling loads are not reasonably balanced over the year, the size of the GHX is larger. If the lighting system contributes much of the heating the building needs in winter, heat isn't extracted from the GHX. If the lighting contributes heat to the building all summer that has to be removed by the GCHP system and then rejected to the ground, to loads become more imbalanced. The GHX becomes larger and more expensive to build.
The imbalance becomes even greater when you consider that the power to operate the heat pumps (the compressors, pumps, fans, etc.) all ends up in the ground when the system is cooling, but contributes to the building heating in winter.
A designer can use this to his or her advantage. In a building that is heating dominant, it might be advantageous to specify inefficient lighting, as the lighting basically becomes a source of auxiliary heat for the building. Alternately, more efficient lighting forces the GHX to contribute more heat to the building in winter and reject less heat to the ground in summer.
Lighting system controls such as occupancy sensors, and daylight sensors can help control the amount of heat the mechanical system has to deal with as well...but that's a topic for another day.
In my blog I'll be expressing my opinions about what I've the learned about ground coupled heat pump (GCHP) systems over the last 30 years. I've been very fortunate to work with many interesting people who are passionate about this technology...engineers, geologists, mechanical contractors, drillers, excavation contractors...in different parts of the world. I've learned a lot from them and will be using this forum to pass on some of the things I've learned and feel are important. Please feel free to use this information if you feel it's worthwhile...hopefully you can avoid some of the same mistakes I've learned from.
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