How to lengthen the lifespan of Vertical Geothermal Well with Evacuated Tube Solar Collectors in Nordic Climates?
Closed loop geothermal heat pump system is one of the most energy efficient and reliable technology for residential, commercial and institutional buildings. It provide steady thermal energy output that is less dependent on outdoor climate conditions, since ground thermal properties, such as undisturbed ground temperature, are fairly constant below certain depth. When properly designed for its application, geothermal closed loop and water source heat pumps have a longer lifespan that conventional air source heat pumps and require less maintenance.
Like any other technology, Geothermal closed loop has several drawbacks: First it's still more expensive than conventional Heating and Cooling systems, Second it requires specialized skills for designing and drilling geothermal wells and Third (in Nordic Climates) ground temperature around vertical well will drops significantly over time, especially in applications where heat balance (between Cooling and Heating) is not Zero.
The first two drawbacks will not be addressed in this blog. We will explain here how to design a geothermal system with an annual zero heat balance and a maximum lifespan.
In a typical Canadian Heating and Cooling Residential Project, Heat extracted from the Geothermal Wells in Winter is 2 to 3 times the Heat Dumped into the Wells in Summer. Over a longer period of time (10-15 years) Ground Temperature around geothermal wells will drop from Undisturbed Ground Temperature at 10 ⁰C (50 ⁰F) to 0 ⁰C (32 ⁰F).
This reduces significantly the energy efficiency of any geothermal heat pump. After more and more operating years with negative heat balance, ground temperature continue dropping till it becomes lower than the minimum operating temperature of geothermal heat pump. At that time Geothermal Wells become useless for heating application.
Why an unbalanced annual heat balance for geothermal wells?
- Nordic Climate:
In most Canadian Cities (Located South), we have eight month of heating demand with a large temperature difference between indoor and outdoor Temperatures and only two to four months of cooling demand with much smaller temperature difference.
- Building Code Evolution.
Canadian and US building code are becoming more stringent on buildings' envelope energy efficiency, passive cooling, natural ventilation and air tightness. This reduces significantly the cooling demand in summer which tilts the annual energy balance toward heating.
- Domestic Hot Water Pre-Heating:
Most Geothermal Heat Pumps have features for heating or Pre-heating Domestic Hot Water. DHW heating demand is 12 months a year.
- Process Heating Applications:
In some projects process heating applications, such as snow melting, fermentation process, Thermal Decontamination, etc..., requires excessive amount of heat extraction in winter and zero heat dumping in summer.
How to compensate for an unbalanced annual heat balance
To lengthen Geothermal Well's lifespan, we should maintain the balance between heat extracted from the ground and heat dumped into it. This will help keep the ground temperature around the well equal to the undisturbed ground temperature which will maximize the Coefficient of Performance (COP) of Geothermal heat pump connected to the well (s).
Vacuum Tube Solar Collectors (VTSC) is an amazing technology that produces thermal energy in both winter and summer. As explained in our previous blog (Seasonal Underground Thermal Energy Storage), we have longer sunny hours in summer and for that Vacuum Tubes' thermal energy production exceeds instantaneous energy demand. The excess heat in the summer can be dumped back into the geothermal well in order to compensate for the well's thermal energy deficit.
Notes. The non Zero Heating demand summer is associated to either DHW heating or Process Heating. For more info on how Vacuum tube solar collectors work please see this blog: How do Vacuum Tubes Solar Collectors Work?
Dumping excess heat into the geothermal wells can be achieved directly without even running the water source heat pump. When thermal energy produced by Evacuated Tube Solar Collectors is not used, it can be pumped directly into the ground. The Pumped fluid (Usually a Water / Glycol Mix) releases its heat into the ground and goes back to the solar panel to harvest more heat.
How many Evacuated Tube Solar Collectors are needed? Heat Dump deficit from the ground shall be first estimated by running energy simulation. Heat Dumped into the ground should be equal to the deficit in order to maintain the ground temperature at the natural undisturbed ground temperature.
By maintaining a zero heat balance between heat extracted and heat dumped from/to Geothermal well, its lifespan can be extended up to 50 Years and heat pump useful lifespan to 25 years.