Flow conditions for solar heating pits are well-known. The flow conditions for the smart energy application is very case dependent and will add extra work for dimensioning of inlet, outlet and piping arrangements. Fortunately, the circumstances for the utilization of PTES implies also much better economic conditions. As visualized in Figure 2, the right plot, a fictive case with multiple heating cycle is simulated. Each of these cycles can contribute to the payback for the investment. Further additional contribution to the overall economic performance could come from the interaction with the electrical markets that include regulation and flexibility services and the utilization of changing prices. Technologically, the thermal utility sector can offer their thermo-electric systems, such as heat pumps and organic ranking cycle engines. This improved situation is, in the current discussion, exploited for faster payback time, instead of invested into more robust solutions. The author pleads for the latter. ACTUAL PTES PROJECTS Currently, a pit is built at Høje Taastrup, Denmark, where the temperature conditions are defined by waste utilization, hence the future conditions are demanded already now. A similar pit is under consideration in Aalborg and at a few other places in Denmark. The author is involved in pit designs in the Baltic countries, which are very similar to Danish conditions. From this we can conclude that there is a demand for developments. The cases also show that the strict economical conditions, which were the fact for solar thermal plants, have changed together with the requirements. In actual projects in the Middle East and Australia, concepts for hot and cold pits are under design. Also, here, the economical boundaries give reasons to be optimistic to find robust PTES solutions within the very good economic conditions of current solar pits. Recently, US companies have shown interest in this world’s cheapest energy technology. CONCLUSION Conclusively, it can be stated that the operating requirements by the surrounding energy system bring along improved economic conditions for future PTES. This is due to the improved potential per volume and most important the repeated and fast utilization. Hence, it is certain that there will be thermal pit storage that will meet the needs of tomorrow by applying more robust materials, solutions and design principles
In Figure 2, we see three plots, to the left, the measured curves at different depth of the pit during the first two years of utilization. Focusing on the second year, the plot represents a typical seasonal solar production. In the middle plot, the shape of the solar application is changed to the shape that can be expected for future applications in smart energy systems. This shape is then repeated in shorter load and utilization cycles in the right plot, a main parameter in the argumentation below. From the left plot in Figure 2, we find that in the second year, where the surrounding is adjusted to the storage, the top part of the pit is above 80ºC for approximately 3 months of the year. This observation supports the assumption that is applied for the estimation of the lifespan of liners. From this we can conclude that the solar pit will behave according to expectations, and so will the liners, which are expected to function properly for the next 20 years. THE SMART ENERGY SYSTEM REQUIRES CHANGES TO THE PTES TECHNOLOGY A sustainable development currently drives the expansion of wind and photovoltaic energy generation that is characterized by being electrical and strongly fluctuating. The smart grid is addressing the challenges of this development, but later experiences show that it is necessary to include other energy carriers to stabilize the smart grid, resulting in the smart energy system. In all this, energy storage is important. The obvious solution of electrical storage is limited in potentials and expensive. Hence, other energy storage technologies are required. Currently, thermal energy storage, especially PTES, is scalable to very large size with respect to natural resources and is the cheapest technology at hand. However, the technology is designed for the conditions of solar heating and cannot be directly applied to the new requirements without jeopardizing the lifespan of the materials and solutions. For the smart energy system application of PTES, a first change in operation stems from the energy sources, such as CHP, waste heat and the like, that offer much higher temperatures, hence requirements for heat storage near 100ºC are a reasonable compromise. There are two basic ways to go: a) designing the pits for shorter lifespan and improving the exchange of critical components, liners and insulation, or b) going for more robust materials. Both are economically feasible. The former is probably to prefer for the time that is necessary to find more robust solutions. Work is going on to find high temperature polymer liners, metallic liners, double liners and alternative insulation materials. The bottom temperature in solar ponds must be low to avoid overheating of the collector field. This is not the case for the smart energy operation, where the whole pit storage is used for high temperatures. Consequently, this affects not only the top liner, but also the whole pit liner and calls for thermal insulation at the sides and bottom of the pit that are able to meet the high-water pressure and adds cost to the solution.
For further information please contact: Alfred Heller, firstname.lastname@example.org
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