Basic
The walls will be constructed using 2 x 4 lumber and batted fiberglass insulation. They will have 1/2 inch plywood sheeting on the exterior along with vinyl siding. On the interior, the walls will be covered with 1/2 inch drywall.
The ceiling will be constructed using 5/8 inch drywall on the interior along with batted fiberglass insulation between the trusses.
The floor will be constructed using 4 inch thick medium concrete. For the load analysis, only one dimensional conduction will be considered.
Inexpensive windows were selected for use. These windows are not energy efficient. They will be Pella Thermastar windows.
The materials used in the construction of the walls and ceiling were combined in an appropriate manner to determine the U -Value. The table below shows the materials individual U values based on thickness and thermal conductivity.
The materials were combined to obtain an overall U value as shown:
These U values were used along with the exposed areas of each and the design temperature difference for both heating and cooling to obtain total loads for heating and cooling. The results are shown below.
Using the calculated loads, a geothermal system was designed. The geothermal unit, comprised of the indoor and ground loop heat exchangers, the blower, and the compressor will be provided from GeoComfort. The unit will need to be 36,000 Btu nominal rating to handle 80% of the heating load. The performance characteristics given by the manufacturer were used to determine the COP, EER, and necessary flow rates for both the air and the ground loop fluid. The general circuit for the geothermal unit is shown here:
The ground loop was designed using 1" black plastic pipe. The length was set to 150 meters per loop using three loops total. The flow rates and heat extraction of each loop was assumed to be equal. The effectiveness of the ground loop was determined by treating it as a simple heat exchanger where the ground temperature, and therefore the outer wall of the pipe, would be maintained at 50 deg F. The effectiveness was calculated as being equal to 0.90.
Equipment Selected:
Geothermal unit - GeoComfort 036C
Ground Loop Pump - Grundfos UPS 15 - 58F/FC
The nominal output of the geothermal unit during heating includes the work done by the compressor and the work done by the ground loop pump. To get an accurate estimate of the steady-state ground loop temperatures, evaporator and condenser temperatures, and ground loop heat transfer, a simulation was created in Matlab. The following diagram shows the basic layout of the overall system:
The value of q cd was assumed to be the known output of the unit. The return air temperature is taken as the house temperature. The ground temperature is a known value.Unknowns: T supply, T cd, T ev, T3, T4, T5, q ev, qcd
Knowns: T house, T g, q cd, W pump, COP, Ca, Cf, effectiveness of ground loop, effectiveness of evaporator, effectiveness of condensor
Eight equations were developed and were iterated using MatLab. The results of the simulation are shown here:
The cost of operation of the Basic Geothermal Home was calculated using the air exchange rate for the home and the applicable heating and cooling degree days in Iowa. The results are shown here and compared to the operation of a conventional HVAC system that employs a Natural Gas Furnace and split type Air Conditioner.
The payback period has been illustrated for the geothermal system using the upfront difference in cost and the operational savings per year. The following plot indicates the number of years of operation to payback.
Payback Period = 16 Years
