Roger Abdo - February 14 2021
Why and When Closed Loop Horizontal Geothermal fails to deliver promised Energy Savings in Cold Climates?
GHorizontal closed loop geothermal Energy is an affordable yet steady source of thermal energy for small to medium size projects. Despite the fact that it's less efficient than vertical closed loop geothermal it comes with a much lower cost and it's more popular in rural areas where space is not an issue and equipment for trenches excavation are easily available (such as in farms, wineries, rural DIY homeowners, etc....).
When Properly designed and installed, Closed Horizontal Loop combined with a Liquid Source (Geothermal) Heat Pump, delivers significant amount of thermal energy that can be used for Space Heating and Cooling, Domestic Hot Water Heating, Snow Melting, Process Heating, etc.... which can help reduce energy consumption and annual utility bills.
Proper design and sizing of a geothermal heat exchanger requires advanced technical skills that most designers do not have. The majority of designers in the Canadian / US Market rely on rules of thumb that have been used for years such as 3 Tons for every 500 ft. deep of vertical loop or 1 Ton for every 500 Linear Feet of horizontal loop. These rules of thumb are the result of calculations based on certain assumptions. When these assumptions are not applicable, theses rules of thumb become obsolete and lead to failure. Many Horizontal geothermal projects fail, because these assumptions do not simply apply for the project
Why This Blog?
Content. Increase Customer Awareness about Geothermal Energy. Help Customers Prevent spending their money on projects that are doomed to fail.
Context. This blog is not a course on how to size a closed loop geothermal system. it's rather a checklist every homeowner should ask his installer/designer. It simply highlights the most common mistakes geothermal designers make in Nordic Climates.
Structure. The First part of this blog will explains the various variables and parameters influencing the size and behavior of a geothermal system. The Second part will explain the impact of the most critical variable on the whole system. Last Part of this blog will present an action plan or a check list or a guide for a successful project.
Assumptions. For the Purpose of this blog we have assumed a conventional Nordic R-45 Heat Pump with a 1" inground HDPE loop and 10 GPM Loop Flow.
What to Consider When Sizing a horizontal geothermal loop?
Like in any Energy Project, Horizontal Geothermal Loop Length, Depth, Pipe Size, Pipe Type, etc.... depends on many factors such as Nominal Heating/Cooling Load, Annual Heating/Cooling Demand, Soil Type, Soil Thermal Conductivity, Soil Thermal Diffusivity, Soil Temperature Variations, Heat Transfer Fluid, etc.....
Heating and Cooling Load and Load Demand are usually properly calculated by most designers when Space to be heated and cooled is identified. Location specific factors (such as soil properties) can be precisely measured or taken from location geotechnical data.
Heat Transfer Fluid such as Propylene Glycol (PG) influence is usually overlooked by 99% of Horizontal closed loop geothermal designers, since PG thermo-physical properties are relatively stable when PG temperatures is above 0°C (or 32°F). When PG Temperature drops below 0°C (or 32°F), Viscosity Increases exponentially and heat transfer between Circulating Fluid and Ground is greatly reduced.
Showing Propylene Glycol at ambient temperature (Left) and Propylene Glycol at -5°C (or 23°F) (Right)
Heat Transfer Fluid (usually Propylene Glycol) other thermal properties such as Thermal Conductivity, Heating Value and Density vary with fluid temperature. Conductivity and Heating Capacity decrease when temperature decrease and Glycol Concentration increases. Density follow a different pattern, it increases when temperature decrease and Glycol Concentration increases.
First Heat Exchange Indicator Between Heat Transfer Fluid and Ground: Reynold Number
Reynold Number is a Dimensionless Number used in fluid mechanics to determine whether a flow is laminar, transient or turbulent. For a Proper Heat Exchange between PG and Ground, Fluid's Flow inside the geothermal loop shall never be laminar. Both Transient and Turbulent Flow provide a good healthy heat exchange between PG and Ground. The higher the velocity inside the ground loop, the more turbulent the flow is and the better is the heat exchange.
Higher fluid velocity comes with extra cost of higher pumping power consumption. Transient Flow with Reynold Numbers between 2500 and 5000 represent a tradeoff between reducing pumping power and having a good heat exchange between PG and Ground.
Reynold Number is defined as the product of density, velocity and diameter of inground loop divided by the viscosity of fluid circulating inside the loop. The colder the ground is, the higher is the PG viscosity and the lower is the Reynold number.
As we can see in the above Graphs, when Ground temperature drops in winter, heat transfer fluid temperature drops too, viscosity increases and Reynold Number decreases which make flow very laminar and unlikely to extract heat from the ground.
Second Heat Exchange Indicator Between Heat Transfer Fluid and Ground: Inground Heat Exchanger Thermal Resistance
Heat Transfer Potential is expressed by the thermal resistance property. The lower the thermal resistance is, the higher is the heat exchange. Overall thermal resistance between ground and Heat transfer fluid is the sum of the thermal resistance of: Convection Heat Transfer between PG and Loop Pipe, Conduction Heat Transfer between Loop Inner and Outer Layer of Pipe and Conduction Heat Transfer between Loop Outer Layer and Ground. The last two become constants once Pipe is selected (Usually HDPE pipes SDR11 either 3/4 or 1 Inch Diameter) and inground loop location is determined. The only thermal resistance that is influenced by ground temperature, fluid viscosity and velocity is the convection thermal resistance of PG inside the loop.
In light of the above graphs, 40% and 50% PG concentration have little potentials of delivering enough heat exchange when ground temperature drops below 0°C (or 32°F) and 7°C (or 44.6°F) respectively:
1- 40% Concentration PG shall not be used at depths where winter ground temperatures are susceptible of dropping below 0°C (or 32°F).
2- 50% Concentration PG shall not be used at depths where winter ground temperatures are susceptible of dropping below 7°C (or 44.6°F).
Third Heat Exchange Indicator Between Heat Transfer Fluid and Ground: Heat Pump Flow Vs Calculated In Ground Loops Flow
Liquid Source Heat Pump Manufacturers specify condenser flow in products catalogue which might not correspond with the calculated minimum flow of inground geothermal loops. In places where soil is dry, inground loops required flow is usually higher than heat pump required flow, a by-pass is installed between inground loops and heat pump in order to satisfy both flow demand and not to compromise heat exchange between heat transfer fluid and ground.
When bypass is present, pump friction loss shall be sized for the calculated loops flow downstream the bypass and heat pump flow upstream the bypass.
Conclusion and Homeowner Checklist
Assuming Heating and Cooling load have been properly calculated, Geothermal Heat Pump has been properly selected and inground loops length is properly calculated, preventing poor performance of the whole geothermal heat pump / inground loop combination depends on the proper heat exchange between heat transfer fluid and the ground. For that Purpose Ground temperature at inground loop depth must be known as well as soil thermal conductivity and thermal diffusivity:
Ground Temperature at a certain Loop Depth is location Specific, Factors such as Undisturbed Ground Temperature, Thermal Diffusivity of Ground, Annual Temperature Swing, etc.... are used to approximately find ground temperature at the depth of geothermal loop.
For Nordic Climates (Canada and the Northern Part of the US), Loop depth shall be optimized to balance Excavation cost and Loops energy efficiency
1- Never use Propylene Glycol with concentrations exceeding 30% in volume. As seen above 40% Concentration make loop ineffective when ground temperature drops below 0°C (or 32°F).
2- Minimum Loop Depth shall be the one that Minimum Ground Temperature is equal to the Heat Pump Minimum Leaving Fluid Temperature plus 4°C (or 7.2°F). Use the above Graph to calculate ground temperature and heat pump manufacturer technical manual to find out the heat pump leaving fluid temperature.
3- Use a By-Pass between heat pump and inground loops if needed and properly size circulation pump for the calculated friction loss.