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.
John D. Manning, PE