Earth Sensitive Solutions

LOOP FIELD DESIGN - "GROUPING"

2009-08-08T14:33:00.000-07:00

The “Grey” Paper Concept – There is a term widely used called a “White Paper”, I do not know the derivation of this term, but it implies to me that the world can easily be described in black and white terms. Anyone involved in technical design has to recognize the “grey” nature of the design process; a good design is one that embodies the appropriate amount of compromise to strike a balance yielding maximum value. Consequently, the concept of the Geo “Grey” Paper is not so much to present the definitive right (“white”) answer, but to present a discussion of a singular issue that stimulates a rigorous evaluation on the part of design engineers that are now engaging in geothermal system design.

In the broad spectrum of commercial loop fields that have been designed and installed there is great diversity in what I am calling the grouping strategy. Specifically, this grouping strategy is the assembling of multiple vertical bores into a group that is connected to a reverse/return manifold and a single supply and return pipe which is then connected to a valved manifold located in either the mechanical room or a vault (the whole concept of using a vault is a subject for a separate Geo “Grey” Paper). The range of grouping that has been utilized in designs can be grouped into one of three categories:

One Group – This approach uses a single supply and return pipe that is connected to all the vertical bores on the project either in a single reverse return strategy or even multiple reverse return manifolds that are then gathered into one major reverse return gathering manifold.
Multiple Groups – In this approach a number of vertical bores are fed with a reverse return manifold which is connected to supply and return pipe that is then connected to a valved manifold in a mechanical room or a vault. The entire loop field is then comprised of these multiple smaller groups, all connected to the same pair of valved manifolds (one supply and one return).
Home Runs – This concept is simply having each vertical bore be connected to a supply and return pipe that is connected to a valved manifold in a mechanical room or vault.

It is easy to visualize how each of these solutions will have a different impact in a variety of areas in the final product. Specifically, these areas would include the following:

Material Cost – Polyethylene Pipe, Valves, Wall Penetration Seals and Antifreeze
Installation Labor – Pipe Handling, Fusion Joints, Wall Penetrations, Manifold Assembly
Pumping Energy – Pressure Drop
Flow Balance – Flow Balance will impact Loop Performance or require Balance Valves
Loop Field Reliability – Number and type of fusions and mechanical joints
Ease of Flushing and Purging – Required Flush and Purging Apparatus

For the sake of presenting a quantitative discussion, as well as qualitative, I have elected to create a typical project scenario:

Small – (20) 500’ vertical bores with 1 ¼” loops – Each loop will have a design flow of 10 gpm (System Flow = 200 gpm) and the loop field is located 400’ from the mechanical room. System will use 18% Propylene Glycol antifreeze.

This paper will be expanded in the future to quantify in a similar fashion the following scenarios:

Medium – (80) 400’ vertical bores with 1 ¼” loops – Each loop will have a design flow of 8 gpm (System Flow = 640 gpm) and the loop field is 60’ from the mechanical room. System will use 10% Propylene Glycol.
Large – (300) 200’ vertical bores with ¾” loops – Each loop will have a design flow of 3.5 gpm (System Flow = 1,050 gpm) and the field is 100’ from the mechanical room. System will use water only.

This nominal 65 Ton Loop Field is located 400’ from the mechanical room and the various grouping strategies would result in the following design details:

The table above illustrates that the selected pipe size has resulted in each design having generally a similar total pressure drop (between 31.7 and 34.6 Ft). This pressure drop is reasonable for a loop field and has the vertical loop creating the larger portion of the total pressure drop, which will enhance natural flow distribution (eliminating the need for any circuit setter valves on the manifolds). Generally speaking, pressure drop translates to operating costs as well as initial cost associated with a “stronger” pump.

Generally, the following table illustrates material cost differences for both the polyethylene pipe, valves, Metraseals and the antifreeze:

The assessment of impact on installation labor is difficult to make quantitatively, generally speaking the number of fusion joints and the amount of pipe to handle might be helpful to quantify labor, but they have severe weakness, due to the speed of fusion and overall productivity can be dealt with by “tooling up” appropriately, and it is a given that Socket Fusion Tools suitable for 2” & smaller can be purchased for significantly less then butt fusion equipment capable of fusing 6” PE Pipe. Additionally, core drilling is certainly labor intensive, but a linear relationship between number of holes to labor content is not valid, the size of the hole will also determine labor content.

Now, let’s discuss the flushing and purging requirements which can be quantified. The standard in the industry associated with the flushing and purging process is the minimum accepted velocity of 2 feet/sec. Consequently, it is simple to calculate the required flow to achieve this velocity in every section of the loop field to perform acceptable flushing and purging (the process that cleans debris and air out of the system). The following table illustrates the flow and corresponding pressure drop and the Pump Horsepower to achieve this flow and head condition:

To accomplish the flushing and purging of both the Home Run and Multiple Group Designs a standard residential Flush Cart, which has either a 1 ½ or 2 HP Pump will be acceptable. The One Group Design will require a larger Purging Pump, although 5 HP is certainly not an outrageous size and can be acquired and handled without a great deal of difficulty.

The final area of discussion is the overall loop field reliability. There is a general “fear” associated with having the source of all heating and cooling buried underground with no serviceability. This fear is often expressed as the “What if….” questions. “What if a loop fails?” “ What if a fusion joint fails?” “What if there is an earthquake?” “What if a ‘wild backhoe’ eats one of the pipes?” “What if the heat transfer were to stop?”. Granted, some of these questions may be more absurd than others, but frankly, the biggest threat to loop fields is the ‘wild backhoe’.

The second Achille’s Heal is the quality of the fusion joints. It is imperative that quality fusion joints be made, and simply put if the technician is not properly schooled in the “art” of polyethylene fusion then 1 joint in the system is too many and the long term loop field reliability is at risk. And conversely, if the technician is properly qualified to do this work then the failure rate is so incredibly small that the loop field reliability is unchanged whether there is 10 fusion joints or 1,000 fusion joints.

Another method of evaluating system reliability is by understanding what the consequences are associated with a “loop failure”. The entire loop field is essentially only required under full load, which occurs by ASHRAE definition 1% of the time. When a part of the loop field is not functioning the temperature difference required between the fluid in the pipe and the earth will adjust proportionately resulting in a different temperature entering the heat pumps then what was designed for the system. Each system and geographical location as well as any imbalance between heating and cooling will influence the degree of risk as to whether or not the actual operation of the heat pumps are at risk. Specifically, in the Northeast on a relatively small system where there is good balance between heating and cooling, the peak summer design temperatures may be 85 F, while the minimum winter temperature may be 35 F. Where the average earth temperature is 52o F this would result in a peak load temperature difference of 17 degrees in heating and a 33 degree difference in cooling. If we were to lose 25% of the loop field then the 17 degree difference would increase to 22.7 degrees resulting in a entering water temperature to the heat pump of 29.3 F under peak heating conditions. Correspondingly, in the cooling mode a 25% loss of loop field will result in a 44 degree difference under peak cooling load conditions or 96 F entering water temperature at the heat pumps. Both 29.3 F and 96 F are within the operational range of geothermal heat pumps and would result in approximately a 4-5% decrease in seasonal efficiency.

It is precisely this loss of loop field performance that makes the home run strategy seem attractive. It indeed makes a loop field more robust and reduces the system performance impact in the event that a loop or fusion joint or a wild backhoe “takes out” a loop. The Home Run approach reduces the percentage loss to a absolute minimum, the Multiple Group approach reduces the risk to a manageable level while not losing sight of the first cost challenge. Finally, the One Group approach certainly exposes the client to the total failure scenario while offering no significant cost benefits.

To summarize, the Multiple Group approach to loop field design offers well managed first costs while maintaining a very robust quality.

Obviously, geothermal loop field design is similar to any other technical design challenge, there is a fundamental art associated with balancing all the deign objectives while developing a strategy the reflects maximum value for the client. Opinions are great, they form the basis of constructive discussion, I have laid out my opinion and look forward to hearing back from those who may have a different opinion.

Sincerely presented,

John D. Manning, PE

President

Earth Sensitive Solutions, LLC

THE MYTHS, LEGENDS AND TRUTHS ABOUT GEOTHERMAL HEAT PUMPS

2007-01-23T07:11:00.001-08:00

As my first BLOG, I've opted to present an article I wrote a while ago, I welcome your comments and feedback.

First of all, kudos to everyone who spoke out in the recent issues of BIG GREEN (Big Green Digest - 05/05/03) regarding Ground Source aka geothermal heat pumps, it is that passion that will drive the changes we are all striving for....

I would like to convey my take on this, if nothing else, interesting technology. Since 1982, I have lived with a system, installed hundreds of systems, sold material on well over a thousand installations, and designed scores of systems, and prior to becoming a geojunky, I received the UTC Design Achievement Award for Heat Pump Design while working at Carrier Corporation. So although I continue to be a student of the technology, the "scar tissue" I have acquired in the past 25 years has provided me with a perspective that is at least a well-founded bias....

To characterize geothermal heat pump systems with a brief discussion about COP's and electric generation efficiency is like trying to describe your mother as "a woman with gray hair"... I don't even know your mother, but I am sure that description does not do her justice.
The geothermal technology is truly multi-dimensional, however, before I embark on a treatise to fully describe the technology and it's place in the marketplace, I would like to support the concept that it is better to reduce demand then to discuss the most efficient heat/cool source. As an example, the Cambria (GOLD - LEED 2.0) Project reflects an approach that reduced demand (compared to typical office space the square feet per ton was about 650 vs 350-400 sq feet per ton for a typical similar office). This demand reduction was accomplished through the architectural elements of incorporating day lighting, efficient artificial lighting controls, and a raised floor air distribution system to name just a few. However, this reduced demand allowed the geothermal system to be downsized and the entire mechanical system was installed for $11/sq foot (without any of those exorbitant utility rebates). So let's talk economics....

ECONOMICS

Perhaps the biggest myth about geothermal is that it does not reflect a good economic choice without rebates... Personally, I am not a big fan of rebates, because they do create an artificial market, where the investments of many businessmen and women are at the mercy and subject to the whims of utility companies. Fortunately, the utility landscape has changed and today the most common rebate/incentives are managed state-wide through a more long term approach through organizations such as the New York State Energy Research Authority (NYSERDA), which is currently offering a pre-qualified rebate of $800/ton.

When discussing economics, it is important to separate the residential market from the commercial market. The HVAC industry in the residential market is plagued with a constantly degrading commodity mentality. There is extreme variation across the country, with Florida being an example of how low it can go. Air conditioning systems with electric resistance heat are being installed for as little as $400 a ton. And in that market I know several contractors who are successful at selling geothermal systems in the residential market albeit at a much higher price tag.

Within, the residential Market there is also a great disparity in the economic realities. Most markets do not have experienced contractors who can price a job with the experience that would allow them to make money without a whole lot of "contingencies" built in. However, I am familiar with contractors who have installed thousands of systems. For example, in Oklahoma, I am familiar with a contractor who is changing out air source heat pumps for $4995 for a 3-ton system, including the loop field and heat pump, and he is making money.
The geothermal "do-it yourself" market is an un-tapped opportunity. I have helped dozens of homeowners, who, with access to a backhoe, were able to install systems for $5-7,000. The development of "Stab" fittings and non-pressurized flow centers has allowed the geothermal technology to be installed almost as easily as installing a washer and dryer (including plumbing and electric hook-ups).

Consequently, it is very difficult to make blanket statements about the economic justification of geothermal HVAC Systems in the residential market.

The commercial market, although very diverse, has a more consistent level of quality and pricing. At the low-end of the spectrum are the gas-electric rooftop equipment/systems, while 4-pipe VAV systems dominate the high end. The typical range for a commercial HVAC System is $8 to $20 per square foot, with local market conditions affecting it up or down. In the past 3 years, all the geothermal projects that I have designed and were installed, fell in the range of $10 to $16.50 per square foot, prior to any rebate. The projects included a dormitory, office building, several schools and a doctor's office. Rebates provided a discount from $2-$4 per square foot. So, I can emphatically state that geothermal is directly competitive with comparable commercially available technology, and anything the architect can do to reduce the load will manifest itself with an even more competitive geothermal option.

But there are many horror stories out there.... Such as a project that went to bid and the result was a geothermal system that came in at $36/sq foot, obviously blew the budget... I would suggest that engineers who elect to design a geothermal project without the benefit of experience are doing a disservice to their client. The danger of poor design is not only a higher first cost, but also operating costs that are way out of line. I actually performed an energy audit on a facility with a geothermal system, and it turned out that the circulating pump was consuming more energy then all the heat pumps combined, the engineer should have been "Tarred & Feathered". The conventional engineering mantra of "2X + 1" can kill a good design before it even gets off the drawing table. Let's go beyond economics....

LOW MAINTENCE/HIGH RELIABILITY

A comment was made as to "How could geothermal heat pumps have lower maintenance costs then regular DX systems that use the same components?" There are two very significant differences in the stress that is experienced by the identical components in two different applications.

The first is the inherent pressure ratios differences associated with air-cooled condensers vs. water-cooled condensers, and the fact that the stress on the compressor is directly related to pressure ratio. An air-cooled DX system will condense at temperatures 25 to 35 degrees above the air temperature, while a water-cooled condenser will be 12 to 15 degrees above the water temperature. Additionally, for a given air conditioning load, the temperature of the fluid in the ground loop is considerably cooler then the air temperature, further reducing the stress on the compressor.

Secondly, when compared to an air-source heat pump, there are no defrost cycles on a water source heat pump. Consequently, the same components will experience significantly different stress levels, thus increasing the inherent life expectancy for a compressor in a water source heat pump. But what about comfort....

COMFORT

Ideally, in the heating mode for forced air systems, the perfect supply air temperature is one that feels warm to the touch, but not so warm as to cause excessive air stratification. Two extreme examples of an uncomfortable supply air temperature are an air-source heat pump that can supply air at such a cool temperature that it can feel drafty and a conventional furnace that can supply heated air at temperatures as high as 140, which will cause stratification and a sense of having a warm head, but cold feet. A geothermal water source heat pump supplies air at a temperature between 95 & 105, which is ideal.

Perhaps the most comfortable form of heat is in-floor radiant heat, which a water-to-water heat pump can deliver at extremely high efficiencies. A COP of 5.0 is easily achieved when producing water at a temperature of 80-90 degrees.

One of the keys to comfortable cooling is humidity management. Cooling without enough dehumidification can result in a clammy feeling, which is certainly uncomfortable. Water source heat pumps have excellent sensible heat ratios, and as a result of a consistent loop temperature can be sized closer to the load requirements, which minimizes the negative effects of short cycling. How can we be comfortable if we are damaging our environment....

ENVIRONMENTAL ISSUES

Using Carbon Dioxide emissions as the key parameter of gauging environmental impact, and comparing the New York State Average CO2 emissions per KWH of .957 lbs/KWH to the embodied CO2 for natural gas of 117 lbs/Mbtu a comparison of geothermal to conventional HVAC systems can be made. With an average COP of 4.0 and a 40% improvement in cooling performance and a Natural Gas combustion efficiency of 82% (and adding fan energy impact of 5.8 lb/Mbtu, which is typically overlooked) a CO2 emission reduction of 47.7% is easily obtained. Being located in Rochester Gas & Electric Service Territory would result in a 62.4% reduction. And the ultimate goal would be to source the electricity from a renewable source and eliminate CO2 emissions by 100%.

As we as a society become better at producing electricity a geothermal installation that is "plugged into the grid" will continue to improve in environmental performance, as opposed to an on site fossil burning technology, which will lock the facility into a given emissions level for the life of the system/facility.

ARCHITECTURAL APPEAL

Show me an architect who appreciates not having to hide outdoor equipment or try and make a chimney look good and I will show you an architect who likes geothermal heat pumps. Numerous historical renovation projects have benefited from the use of geothermal heat pumps because of being able to maintain much of the original character of the building. As I am writing this, Auburn Memorial City Hall is an example of such a project. For 70 years, the employees of the City of Auburn did not have air-conditioning, not even a window AC Unit. Currently a geothermal system is being installed, and by the time the heat of the summer arrives the system will be operational with no visible sign of the system. The citizens of Auburn will be pleased that they are not only efficiently keeping the facility comfortable, but it is being accomplished without compromising the visual appeal of the building.

SUMMARY

Geothermal heat pump systems offer an incredible opportunity, but not just on one front. From simple economics, to comfort, to excellent reliability, to reducing our environmental impact, etc., this technology is commercially available and is applicable on virtually all types of applications, including underground electrical vaults, to ice rinks, to office buildings, to schools, to historical projects, to low income housing, to hospitals to name just a few.
I look forward to hearing/reading your comments.

Respectfully submitted,

John D. Manning, PE
Earth Sensitive Solutions, LLC
PO Box 3; Skaneateles, NY 13152
P: 315-253-3779 / F: 978-285-5876

“There are in fact four very different stumbling blocks in the way of grasping the truth, which hinder every man however learned, and scarcely allow anyone to win a clear title to wisdom, namely, the example of weak and unworthy authority, longstanding custom, the feeling of the ignorant crowd, and the hiding of our own ignorance while making a display of our apparent knowledge.”
-- Roger Bacon