Techno Solution or Fad?
Ground source heat pumps (geo-exchange heat pumps) have their place as an effective heating/cooling plant solution, and are a great application in some parts of the country. But if I was building a house and had to decide whether to put $15,000...
Ground source heat pumps (geo-exchange heat pumps) have their place as an effective heating/cooling plant solution, and are a great application in some parts of the country. But if I was building a house and had to decide whether to put $15,000 into a ground source heat pump or into more insulation, better windows, and a more carefully integrated design, I wouldn’t do the heat pump.
The analogy is the same no matter what size or type of building is involved. The first thing that needs to be done in terms of low energy building design is to design a proper envelope and engage as much passive design as possible before the techno-solutions are applied. Future-proof the building first. After all, the most efficient heating and cooling system is the one that doesn’t have to operate.
Geo-exchange heat pumps are a popular HVAC system these days due to the growing number of LEED registered buildings and the “apparent” energy savings the geo-exchange system can provide to get those highly desired LEED energy credits.
But while ground source heat pump systems (GSHPs) can show they can provide more heat energy than the electrical energy going into them, there are other matters to consider in order to see the big picture of truly sustainable and environmentally friendly building system designs:
- Where is the electrical energy that powers the heat pump coming from, and how is it being generated?
- How efficient are the building terminal systems in terms of transferring the geo-exchange plant heating and cooling energy to the occupied space?
- How does the current method of building energy modelling create unrealistic expectations compared to the actual reality?
The overall system is what counts
Throughout this article I’m referring to the overall system energy efficiency of a geo-exchange heat pump system. Most people are excited about the high coefficients of performance (COP) of the water-to-water heat pump unit. A proper engineering evaluation, however, must also include all the system electrical energy loads — including the ground source circulating pump’s energy. When all the system energy loads are taken into account, the actual overall COP of a well designed geo-exchange heat pump system seldom gets a seasonal “system” efficiency above 2.5 to 2.75, even with the heat pump unit itself having a COP of 4.0 or more.1 Again, this is a general condition, and the actual COP will change depending on the specific design, location, and source side temperature conditions that are present.
It is true that heat pumps and their high COPs and energy efficiency ratios (EERs) can show significant energy cost savings, a factor that is being hyped pretty well all over the internet and at every green energy trade show. But cost savings compared to what? Compared to a brand new well-designed conventional system in a very well-designed building?2 Or compared to replacing a 20-year old conventional HVAC system in a poorly insulated building? What, exactly IS the baseline?
I have just read an article in the local news stating that BC Hydro is applying to raise the electricity rates 32% to 50% over the next three to five years. Meanwhile, natural gas rates from the local utility, FortisBC (formally known as Terasen), have fallen over the last five years and have flattened out for the last year. What effect do you you suppose the price changes will have on all the energy cost savings that models for heat pump systems showed in the last three years?
If overall carbon emissions are not a concern, and you want to evaluate a geo-exchange heat pump system against another system but based only on energy costs, this will be a very specific regional evaluation that may work in areas where there are low electrical rates compared to fossil fuel energy costs, but not in many other areas of North America. The point is that there is no common ground to be able to authoritatively say that a geo-exchange heat pump system will save energy costs “everywhere” they are applied. One rule to remember is that “energy costs” are not the same as straight energy use.
In my opinion, there are two basic criteria that should be used to evaluate a building mechanical system and mechanical plant equipment for a low energy, sustainable building design:
- Energy performance (not energy cost)
- Carbon emissions.
Good engineering must balance field source and load temperatures
When evaluating the energy performance of a geo-exchange heat pump system, key factors to consider are the source-side temperature conditions over the whole year, and the load side emitter temperatures and efficiencies to transfer heating and cooling effectively to the occupied space. As the source side temperature differences relative to the load side temperature differences get closer and closer, the heat pump system COP and EER rise significantly. But keep in mind that when the heat pump is in heating mode the COP drops as the source side (geo-exchange side) temperatures get colder. This drop in energy efficiency can be drastic if the geo-exchange field has been sized improperly or the soil conductivity has been miscalculated — a serious design issue for heating dominated climates like most of Canada.
The key to safely taking advantage of the operating energy and possible carbon reduction efficiency of a ground source water-to-water heat pump system, is to use very low temperature heating water and higher temperature chilled water in the building HVAC systems, with a good source (ground) temperature over the whole year.
Here’s where many folks get into trouble. Trying to apply a geo-exchange water-to-water heat pump system to more conventional building HVAC terminal systems that are designed to use higher temperature heating water and lower temperature chilled water seriously erodes the energy advantage of the heat pump system. The other big issue is the soil conductivity and ground heat exchange conditions assumptions and calculations. All of the heat pump system EER and COP estimates can go out of the window if the actual operating conditions of the soils cause the ground conditions around the buried geo-exchange piping to become over-heated in the summer and over-cooled in the winter. Ground temperature problems are also common with geo-exchange ground loops that are designed too small in an effort to keep installation costs down.
The carbon reduction capability of a ground source heat pump system is also complex. Since the North American electrical grid is so interconnected, there is not much difference in the carbon emissions intensity of the source electricity on a regional basis. If you were to make a detailed study of geo-exchange heat pumps and their “system” carbon footprint versus a condensing natural gas boiler system – both serving identical low temperature heating systems, and strictly having minimizing carbon as a goal – the net difference in favour of the geo-exchange heat pump system is small, and fades to insignificant as the building energy losses and gains are minimized by a very high performance envelope and passive building design principles.3
LEED reference model is not true comparison
The other major driver that creates the belief that geo-exchange heat pumps are the golden goose is the building energy modeling process and rules imposed by the LEED and ASHRAE 90.1 energy performance path criteria. The basic problem with the Reference Building versus the Design Building modeling set-up is that a lot of the passive building design elements are not credited. The ASHRAE 90.1 energy modeling path is used mainly to compare systems and equipment performance criteria.
The energy modeling rules require default prescriptive systems and equipment to be used for the Reference Building, but these are not necessarily “business as usual” building systems that woul
d be used for that particular building, in that particular location. And if the Proposed Building Design Model has already incorporated best solar orientation, external sun shading, and an articulated façade to reduce the building heating, cooling, and lighting loads based on passive design approaches, the Reference Building Model must also include the same basic configurations. So let’s get this straight – if we use the proper integrated design team approach to incorporate as many passive design approaches as possible before we even think about what kind of energy efficient HVAC systems we want to apply, we don’t get much, if any, credit for all that passive design!! So what IS the baseline business as usual building energy efficiency we are trying to compare our proposed design to? Well, there isn’t one if you use the commonly applied ASHRAE 90.1 Reference Building method for energy modeling!
This energy modelling method results in an exaggeration of the building energy improvements using a geo-exchange heat pump plant compared to a well-designed “business as usual” alternate/default HVAC system. The drive to seek LEED energy credits leads to the following analogy.
The Humvee vs. the Toyota
You can take a Humvee that gets 15 miles per gallon in stock form, then tune it up and improve its performance so that it gets 20 miles per gallon. That’s a 30% energy efficiency improvement, so assign 4 LEED energy credits for that accomplishment. Now take a Toyota Yaris that gets 35 mpg in stock form, and tune it up and improve its performance so it now gets 45.5 mpg, a 30% improvement in energy efficiency. Guess what? – 4 LEED energy credits for that.
There is no credit for starting off with an energy-efficient design in the first place. Both vehicles have four wheels, air conditioning, and a great stereo and will get you from point A to point B in the same amount of time. No wonder LEED buildings don’t seem to save very much energy compared to the accumulated average of all the contemporary buildings that are built “to Code.” You end up comparing a bad building design (Reference Baseline) with a not quite so bad building design to achieve “energy reductions,” and some LEED Credits.
With buildings — to use an extreme case — if one designs a PassivHaus standard building, such that the heating and cooling requirements can be met by low-temperature heating water, and high temperature chilled water, (ideal for a water source heat pump unit, eh?), you can use a small condensing boiler, heat recovery ventilator, and just direct ground coils, to obtain cool water for building cooling purposes (or air intake earth tubes or natural ventilation in the right climates). This approach could be as energy-efficient, or more energy efficient, than a geo-exchange heat pump plant on a yearly basis, and could result in less carbon emissions. If the electricity to run a geo-exchange heat pump plant comes from a renewable energy source generator, however, then the evaluation can take a very different direction in favour of the heat pump.
Remember, the most efficient HVAC system is the one that doesn’t have to run. Build tight and ventilate right. cce
Geoff McDonell, P.Eng., LEED AP, is with Cobalt Engineering of Vancouver. He has over 30 years of experience in mechanical engineering design, HVAC, plumbing, fire protection, controls systems and passive building design. E-mail email@example.com