Results and Discussion

Results are presented for initial pressures of 164.7 and 200 psia. The conditions are intended to replicate working conditions of a prototype at UOIT, as well as planned future extensions to higher pressure units. In modeling the unit, heat transfer within the pressure vessels and pressure changes within the piston-cylinder device are included.

Table 14.5 System performance with a 40 C (72 F) temperature difference at 3,000 psi.

Temperature of heat source (oC/ oF)


Temperature of cold sink (oC/oF)


48 gallon tank configuration (2 pairs)

Overall heat transfer coefficient, U (Btu/h ft2 F)


Heat transfer rate, Q (Btu/h)*(per tank)


Daily heat transfer, for 24 h/day operation (Btu)


Operating pressure (psig)


Carnot efficiency (%)


Projected hourly output for 4-tank configuration, each tank having 48 gal.


(kW h)

Projected daily output for 4-tank configuration, for 10 h/day operation (kW h)


Revenue for 10 h operation at $110/MW h ($)


Annual revenue for 10 h operation/day at $110/MW h ($)


432 gallon tank configuration (8 tanks)

Overall heat transfer coefficient, U (Btu/h ft2 F)


Heat transfer rate, Q (Btu/h)*


Operating pressure (psig)


Carnot efficiency (%)


Projected hourly output for 8-pair tank configuration, each tank having 432


gal. at 50% efficiency (kW h)

Projected daily output for 8-pair tank configuration, for 10 h/day operation


(kW h)

Projected daily output for 8-pair tank configuration, operating 10 h/day high


output and 14 h/day low output (kW h)

Projected hourly output for 8-pair tank configuration, for 24 h/day steady op-


eration (kW h)

Revenue per day at $110/MW h for 10 h/day high output and 14 h/day low


output ($)

Revenue per day at $110/MW h for 24 h/day steady operation ($)


Revenue per annum at $110/MW h for 24 h steady operation ($)


* The heat transfer rate is evaluated as Q = UAA.T.

In the lab prototype, the current experimental setup includes a thermal source, thermal sink, four pressure vessels (each containing a helical coil heat exchanger), and a system of pipes and valves to direct the flows. Mechanical energy is extracted from the pressure differential by means of the piston assembly. The device is comprised of two sealed compartments and a rack and pinion gear assembly.

A pressure differential between the two sealed compartments causes the piston to move, which in turn rotates the gear. By directing the flow of pressurized gases from two pressure vessels into the appropriate sealed compartments, mechanical energy is produced in the gear system of the lab prototype.

The model is used to predict the power output of the device. Results are obtained on a per-stroke basis and simulations are performed for several pressure vessel sizes and initial pressures. For all cases, a time delay of 0.2 s per stroke is included, accounting for the time required for the valve configuration to change. The results are summarized in Tables 14.1-14.3. Based on the results of the ther-modynamic models, several observations can be made regarding the performance of the device. Concerning the heat exchangers, the induced pressure difference resulting from heating or cooling in the pressure vessels increases significantly when the magnitude of the difference between the heat exchanger working fluid temperature and the gas temperature increases. By increasing the temperature difference between the water and gas, a significant pressure difference can be generated. Using the parameters of the current test unit with an initial pressure of 164.7 psia, a source temperature of 113°F and a sink temperature of 50°F, a pressure differential of 14.6 psia between the two pressure vessels can be generated after a period of 60 s.

The model can be extended to higher or lower temperature differences. For the temperature differences possible with cogeneration or solar thermal panels as heat sources, for example, large temperature differences can be generated. With a source temperature of 167°F and a sink temperature of 32°F, a pressure difference of 33.7 psia is predicted.

Using the heat exchanger model to predict thermal performance with higher initial pressures also yields promising results. By increasing the initial pressure in the tank, there is a dramatic increase in the induced pressure difference, due to a temperature difference arising over a 60 s period. This result demonstrates that system operation at a higher initial pressure will produce a significantly larger pressure difference, and thus a larger energy output. The model underestimates the time required between each stroke, meaning it could take longer for the pressure difference between vessels to equalize than predicted. In an actual system, some energy is dissipated within the flywheel, thereby allowing for a slight increase in delay between strokes.

The results demonstrate the benefit of operating the system at higher pressures. The total power output of the system increases significantly if the initial pressure in the tank is increased. With higher initial pressures, a greater pressure difference results from the same temperature difference. This result has important implications for the design of future generation units, as increasing the initial pres sure would increase the power output of the system, although equipment costs would also be increased.

Figure 14.5 illustrates a multiple pair configuration with piston assemblies for each pair of tanks. This design overcomes the stroke disparity of the single piston design between tank changes. Each pair of tanks in the multiple unit design will have sequential stroke timing and be connected to a common driveshaft which will result in a steady stroke power value to the flywheel and ultimately the generator.

By supporting the use of renewable energy resources and waste heat and a reduction in fossil fuel utilization, the device has the potential to reduce greenhouse gas emissions significantly. Hence, the device may serve a meaningful role in efforts to mitigate global warming and climate change.

Fig. 14.5 Multiple tank configuration.

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook

Post a comment