Tech Talk - Geothermal Plant Opens

The Missouri University of Science & Technology Geothermal system was officially opened last Thursday, some months after the coal and wood fired power plant that had previously warmed the campus had been shut down.

Figure 1. Chancellor Schrader cutting the ribbon to officially open the system.

The operation ended up being a little larger than originally anticipated, although the receipt of several grants kept the need for external funding bonds down to $30 million. Overall, as the old heating and cooling system was replaced around campus, deferred maintenance costs of some $60 million disappeared as the new system eliminated those needs, and is anticipated to generate fuel overall savings of some $1 million initially rising to $2.8 million a year as future fuel prices rise over the years.

In the end some 645 wells were drilled to feed three different geothermal plants located around the campus. Well depths ranged from 420 to 440 ft., and with a background temperature around the wells averaging around 60 deg F.

The installed system is, to a large extent, computer controlled, so that it was necessary to find employment for the fifteen workers at the power plant who would otherwise have been laid off. Given that some took retirement, the University was able to absorb the rest into the workforce in various ways. But it does point out that, now that the system is installed, the number of jobs associated with this new sustainable energy system are significantly below that required at the power plant, and the coal mine and forestry products supplier that previously supplied the fuel. Maintenance of the system, which is largely built around pumps, pipes and valves can, in the main, be carried out by the normal trades staff at the campus.

Figure 2. Overview board for the individual geothermal flow loops

To illustrate the degree of control that the new system exerts on the Heating and Air Conditioning (HAC) network, consider a simplified circuit for one building.

Figure 3. Illustrated circuit for a single building

Hot water is fed into the building from the network (top left) at a temperature of 118.7 degF, and is mixed with a portion of the previously circulated fluid to give a starting temperature of 113.6 degF entering the building. (The values are in the small boxes over the sensing valve emulations). The hot water circulates around the building providing heat as needed. At the point where the water would exit back to the network for reheating the temperature of the returning water is measured (in this case 102.3 degF). Depending on that temperature a control valve opens or closes to send more (or less) water back for reheating, while the remainder stays in the circuit, with make-up from the main network. (with the valve 41.3% open some 3% of the returning water is being recycled). The computer also calculates the heating load being fed to the building (327.5 kBtu/hr).

Figure 4. Details of the control valve and instrumented values.

By using a similar circuit for cooling the components of the system are largely similar, reducing the inventory costs for maintenance supplies, and the two circuits are simply monitored through instrumentation around the circuit.

This is similarly true for the three geothermal plants, the status of each of which is also represented by a monitoring screen.

Figure 5. Control circuit monitoring the performance of the heat exchangers between the field circulation water and that being used in the building circuit.

The heat exchanges between the ground water and the heating/cooling circuits is through use of three screw type heat recovery chillers, the operation of which is described as:

A heat recovery chiller operates on the basis of a refrigeration cycle: the same basic cycle that is used for refrigerators, air conditioners, and heat pumps you find in your homes. It is designed to provide both useful cooling and useful heating energy from the machine. The work or energy put into the machine through the compressor is used to simply transfer heat from evaporator to the condenser, which makes it a more efficient use of energy than combusting fuel for heat.

As seen in the diagram below, the refrigerate, R-134a in our chiller, is first compressed using a screw-type compressor. This hot gas is then condensed to a liquid as it travels in a circuit through the condenser, and heat is transferred to the water flowing through the condenser tube bundle. The pressure and temperature of the refrigerant is reduced as it flows through the throttling valve. The refrigerant next passes through the evaporator where heat is transferred from the water flowing through the evaporator tube bundle back to the refrigerant. Then the cycle repeats as the refrigerant goes back to the compressor. The refrigerant is confined inside of the heat pump chiller for the entire process.

Figure 6. Operation of the heat exchanger.

Figure 7. Overview of the three chiller units in the McNutt plant

Manually readable gages provide back-up to the computer monitoring instruments.

Figure 8. Monitoring gages for the chilled water loop.

When additional heat is needed, this is provided by a bank of natural gas heaters for the water that can be engaged as needed, and that are similarly monitored.

Figure 9. Overall monitoring board for the natural gas boiler system

While the system may get an early test of effectiveness this week as a Polar Vortex brings an early taste of winter to town, with temperatures predicted to drop to a high of 34 and a low of 19 on Thursday.

Figure 10. Natural gas boiler to provide additional heat as needed.

Since I won't be able to take advantage of those boilers, I’m glad I have my wood stacked, and that I swept my chimney this morning.