Making Sense of Geo-Exchange

I want to share with my fellow engineers some information on the profitability of using energy from a vast reservoir that is embedded right under our feet, in the ground – low temperature geothermal energy.

Many building owners and designers ask themselves if a ground source heat pump is profitable or if it is only a concept for the future.

The answer depends on where you are located and what is the relative cost of the different energy sources available in your area. Another very important factor to consider is how the system will be used.

Let’s begin with a simple hot water heating system. Assume, for example, that the hot water usage of a building is in the range of 11,300 litres daily. The building could be a fitness centre with a steady occupancy by members so the hot water heating could be averaged for each hour of the day. The system has a water storage tank of 11,300 litres. If the water has to be heated from 4ºC to 60ºC, the average heating requirement (power) will be 30 kW (steady state kW during each hour). This heating power has to be on 24 hours a day, 365 days a year. The annual energy usage will be 262,800 kWh per year.

In the province of Quebec, for a continuous demand at tariff “M” for an average size building (more than 100 kW for the monthly demand), the electricity rate will be in the range of $0.073/kWh (including sales taxes). If the hot water is heated with a standard electric boiler, the annual cost of heating it will be in the range of $19,184 per year.

If the same amount of hot water is heated with a water-to-water ground source heat pump (geothermal system), with a total COP of 3.2, the annual energy cost will be in the range of $5,995. This represents an annual saving of $13,189. The added capital costs of the geothermal system could be in the range of $50,000. With these numbers, the return on investment (simple payback) is $50,000/$13,189 = 3.79 years. These figures are for an optimum return on investment with a system in use every hour of the year. This type of usage profile is not frequent, but it can be found.

In Quebec, if the hot water is heated with natural gas, at an average cost of $0.55 per cubic metre and a seasonal boiler efficiency of 70%, the annual cost would be $19,619. This is similar to the cost of electricity, when heating is on a steady basis.

If the natural gas system uses a condensing boiler, then the annual cost could be lower. For a 90% efficient boiler, the annual cost could be $15,260. The added capital cost for a condensing boiler installation may be $5,000 more expensive than a standard boiler system. The return on investment of the condensing boiler system is $5,000/$4,359 = 1.14 year. It is interesting but it is not the most efficient system.

Using the condensing boiler as the point of comparison, the added capital cost of the geothermal energy system is now $45,000. The annual operating saving is $9,265. Then the return on investment for the geothermal energy system is now $45,000/$9,265 = 4.85 years.

For a site with average ground conductivity and ground diffusion, the geothermal energy system of 30 kW requires the use of three (3.25 exact calculation) wells with a dept of 152 m.

The above example is almost a perfect case for the geothermal energy system. The usage factor is high, the cost of electricity is low, and the alternative energy source has a high cost. This situation could be suitable for the provinces of Quebec, Manitoba or British Columbia, but this is certainly not the case for Ontario, Alberta or any province with natural gas available at a very low cost when compared to electricity.

Let’s now discuss the usage factor for building heating. This is more complicated because the heating requirement is driven by many interrelated components of the whole building. These components are: heat losses of the envelope, heating of outside ventilation air, heat gains from the sun, inside heat gains from the use of equipment, inside heat gains from the use of lighting, the schedules of operation and occupancy of the building, etc. Taken together these factors are what we call the thermal balance.

It is the thermal balance of a building that dictates its hourly heating requirements. Of course, the design of heating, ventilating and air-conditioning (HVAC) systems also has a major impact on the thermal balance of a building.

Figure 1 shows the hourly heating requirements for a hypothetical building located in Montreal. It should be noted that the internal gains associated with the building usage and the gains associated with the outside weather conditions are already taken into account. On the X axis are the annual hours and on the Y axis are the hourly heating requirements in kW.

On this figure, one can see that the maximum heating requirement is 950 kW at hour 368 in the month of January. The number is based on the weather data used for the modeling of the building.

The annual heating requirements of this building have been established at 1,328,000 kWh, with a peak demand of 950 kW. It would be highly expensive to design a ground source heating system to respond to the highest heating peak. All heating requirements over 600 kW are needed for only few hours in a year.

The cost effectiveness of a geothermal energy system could be evaluated by analyzing the thermal balance of the building. A lower kW for heating leads to a larger usage factor. The optimization of the geothermal system also takes into account the cooling contribution, but the heating contribution is more important. For most buildings, the most important energy saving with a ground source heat pump is in the heating contribution. For an average size commercial building in Montreal, the cooling energy is not a major part of the building requirements. It is in the range of 10%.

Figure 2 shows what contribution a 300 kW geothermal energy system makes to the building’s heating requirements. It should be noted that 300 kW represents 32% of the peak heating demand. With the hourly analysis, the graph shows that the geothermal system of 300 kW could supply 81% of the annual heating requirements.

The contribution to the heating requirements by the geothermal energy heat pump for this building is 1,070,000 kWh. For a building with this thermal balance, the return on investment, considering the cost of the several energy sources in the Montreal area, would be between 7 to 12 years.

Figure 3 shows the residual heating requirements that could be supplied by other energy sources (electricity, natural gas, propane, oil, etc.).

Several questions arise from geothermal energy system users about the availability of heat from the ground. In the Montreal area, for a building that is air cooled in summer (air-conditioned) with an average heating requirement in winter, a balance of the heat occurs between the heat extraction in winter and the heat rejection in summer. In this case, the ground acts as seasonal heat storage. In southern Canada, we have this beneficial situation.

What happens when a building is not cooled in summer or barely cooled? In theory, the ground temperature should lower every year and, after several years the geothermal system’s contribution to heating will be reduced. Several analyses of the heat balance of existing buildings using specialized softwares (GS-2000, GLD, Transys, etc.) show a decrease in the performance of the ground source heat pump in these circumstances.

However, in practice it has been found that installed systems for these buildings r
emain operational without a decrease of the ground temperature. This is based on the monitoring of several buildings that have a thermal imbalance. The heat extraction is much more stable compared to heat rejection, where the hot surface tends to dry the ground and lower the heat conductivity.

The difference between the theoretical calculations and the experience in the field could be caused by underground water that migrates and improves the heat sink. Several articles published in Switzerland and Germany suggest this phenomena.

Facing these uncertainties, it is advisable to prepare space for installing solar collectors that can be used to replenish the heat in the ground when heating requirements are more important than heat rejection. These solar collectors could be installed in a subsequent phase only if underground water migration is not present and the performance of the geothermal system deteriorates.

In designing a geothermal energy system, questions might arise about the ground conductivity and ground heat diffusion. Most of the time, the average conductivities of the soil materials can be used, but it is advisable to drill a test well to identify the type of soil before designing the whole system. Tests of the conductivity of the soil could also be used to optimize the number of wells when a large number (more than 20) is required.

In conclusion, the geothermal energy extracted by ground source heat pumps does help to lower the energy usage of buildings. The Ecole du Tournant in St-Constant (see photo p. 27) is an example of well-managed energy usage. It uses ground source heat pumps for heating and air-conditioning and has an annual total energy requirement of 71 kWh/m² per year. It is one of the most energy-efficient buildings in Canada. This school was built in 2001, the architect was Vincent Leclerc, and Dessau was the mechanical and electrical designer. cce