The second principle concerns the layout of the lid. The lid is divided into smaller sections. The sectioning can be seen on the photo of the new lid installed in Marstal, Denmark (figure 2). The lid sectioning has several advantages as it addresses and solves issues with water ponding, air accumulation, and thermal expansion. Each section is designed with an inward fall towards the center using a well-defined ballast layer of gravel. This results in a very efficient handling of rainwater that flows towards the center of each section. In the center of each section, a pump well ensures the removal of the water. This system makes it possible to drain the surface of the lid safely and automatically with no maintenance. Also, any trapped air released from the water will automatically vent at the section's higher level. This can be seen in figure 1. Another advantage of the sectionized lid is that both leakage detection and seeking is highly improved. Any potential leakage in a section will be led towards the pump well, and the control system will detect abnormal events. This means that water cannot accumulate inside the lid and cause the lid to break down. Finally, the sectioning also means that the PTES lid and the PTES technology, in general, are scalable. Therefore, much larger surface areas can be handled, as the number of sections will be increased. State of technology and scaling up With the latest achievements regarding lid design and the Danish market's accumulated experience over the past ten years, PTES is considered a mature technology. A PTES offers low-cost energy storage. It has a long service life, and it can be operated with a minimum of operational resources. The PTES technology, including the new lid design, is commercially available, and a project is currently being installed in Høje Taastrup near Copenhagen, Denmark. Aalborg CSP will supply and install an 11,000 m 2 insulating lid on a 70,000 m 3 PTES with a constant temperature of 90°C in the top of the storage. The PTES technology is a mature technology - also for larger storage. Currently, a tender phase is ongoing for a PTES system consisting of 2 x 500,000 m 3 storages in Aalborg, Denmark, with a design service life of 30 years. The PTES project in Høje Taastrup is a part of the "FLEX_TES" project supported by the Danish research and development program EUDP. Participants in the "FLEX_TES" project are VEKS, Høje Taastrup District Heating Company, PlanEnergi, EA Energy Analysis, and DTU Civil Engineering. The project has been presented in an article in Hot Cool 04/2019.
The scale is one factor of success for the PTES, meaning that a larger scale reduces the cost. On the other hand, the scale has also been one of the challenging factors. Existing PTES systems have surface areas of the insulating lid in the region of 10,000- 20,000 m 2 . The surface's size gives rise to some challenges, and future PTES systems are expected to be considerably larger. The technical difficulties regarding the PTES lid can be divided into four main areas: 1) Water inside lid The water inside the lid due to water vapour diffusion and potential leakages can cause increased heat loss. Polymer liners are not 100% water vapour tight. At high temperatures, the water vapour diffusion through the liner can cause water accumulation inside the lid. 2) Water ponding on top of the lid Handling rainwater on the surface of the lid is a challenge and can result in water ponding and high maintenance cost. 3) Air accumulation below the lid Air relief from the heated water in the storage can create problematic air accumulations below the lid. 4) Thermal expansion The temperature difference of the water from cold to hot condition leads to dimensional changes of the materials used in the PTES. Thermal expansion of liner and insulation of the lid is substantial on a large surface area. If this is not handled properly it stresses the materials. New PTES lid design The technical challenges of PTES lids have been addressed in a development project. The project aimed to create a new lid design that would solve the four main technological challenges. This unique lid design was recently presented and installed at Marstal Fjernvarme in Denmark. The lid design is patented by Aalborg CSP.
The new lid design is based on two main principles.
Firstly, the lid is based on a reversed roof principle, as it is designed as a diffusion-open top cover construction. The purpose of this is to address and solve the issue regarding water accumulation inside the lid due to water vapour diffusion through the liner. In a reversed roof principle, it is ensured that the moisture entering the lid can diffuse out of the lid. Simulations and testing have verified this. Additionally, the roof foil and insulation consist of a multilayer construction. Each layer is designed to the specific environment concerning temperature, diffusion, compression, stiffness, and insulation properties in the construction.
For further information please contact: Morten Vang Bobach, mvb@aalborgcsp.com
Figure 2: The new lid design (10,000 m 2 ) installed on a 75,000 m 3 PTES in Marstal, Denmark.
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