by Linda Bertelsen
Pit heat storage, Hoje Taastrup, Greater Copenhagen, DK Photo by VEKS

Implementing a Pit Thermal Energy Storage (PTES) in an energy system has substantial benefits. In recent years, investments have been made into low-temperature heat storage to develop, optimize, and commercialize the PTES technology. The latest achievements in improving the insulated PTES lid cover have also matured the technology and are scalable. The technology has now reached a state where utility companies consider the technology bankable and are prepared to invest on a larger scale. PTES projects, with storage capacities of up to 1 million m3 each, are currently in the planning and tendering phases.

By Morten Vang Bobach, Product Manager, Senior Engineer, Aalborg CSP

Photo above: Pit thermal storage in Høje Taastrup, Greater Copenhagen Area, DK. Photo by VEKS

Basic PTES design

The basics of a PTES are straightforward, as illustrated in Figure 1. A large pit, which will be the storage, is excavated in the ground. The excavated soil is then placed as embankments around the pit. Ultimately, the result is a water reservoir partly below and partly above terrain level. The reservoir is typically lined with a polymer liner before water filling. The top surface is covered with a floating, insulating lid to minimize heat loss. Heat is transferred to and from the storage through pipes and diffusers in the same principle as traditional steel tank thermal storage.

Figure 1 Basic PTES design

Figure 1 Basic PTES design

Benefits of PTES

Thermal Energy Storage (TES) is one of the essential components in the future energy sector. It is a crucial element in reaching the next decades’ global environmental, energy, and climate targets. The TES is the component that binds an integrated and flexible energy system together. PTES is the most flexible and cost-efficient TES. Its high flexibility and low cost enable a cost-efficient transition to a renewable and energy-efficient future. The storage can decouple production and demand, stabilize the energy system, and minimize expensive peak load production. This means a higher share of renewable or excess energy can be utilized. A PTES can serve as a district heating system’s peak load or reserve capacity. Combined with power-to-heat units, it can also help the electricity system by offering up or down-regulation capacity. It enables flexible and efficient power and district heating sector coupling. Moreover, it will be an essential part of Power-to-X (PtX) processes to secure an optimized operation with high total system efficiency.

Technical challenges for lid solution

As mentioned, the PTES technology is a crucial enabler for today’s energy system’s cost-efficient transition. Therefore, the PTES must be robust and extensive with low maintenance requirements.

PTES technology has been developed and refined throughout the past 30 years. In recent years, increased attention has been paid to the design of the insulating lid. Historically, the lid has caused severe technical failures and unwanted thermal losses.

The scale is one factor of success for the PTES, meaning that a more significant 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 10,000-20,000 m2 region. The surface’s size gives rise to 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 costs.

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 conditions leads to dimensional changes in the materials used in the PTES. Thermal expansion of the liner and insulation of the lid is substantial on a large surface area. If this is not handled correctly, it stresses the materials.

Thermal expansion of the liner and insulation of the lid is substantial on a large surface area. 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 to solve the four main technological challenges. This unique lid design was recently presented and installed in Marstal Fjernvarme in Denmark. Aalborg CSP patents the lid design.

The new lid design is based on two primary principles.

Firstly, the lid is based on a reversed roof principle, designed as a diffusion-open top cover construction. This aims to address and solve the issue regarding water accumulation inside the lid due to water vapour diffusion through the liner. A reversed roof principle ensures 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 for the specific environment concerning the construction’s temperature, diffusion, compression, stiffness, and insulation properties.

The second principle concerns the layout of the lid. The lid is divided into smaller sections. The sectioning can be seen on the new lid photo in Marstal, Denmark (figure 2).

Figure 2 Photo of the new lid (10,000 m2) installed on a 75,000 m3 PTES in Marstal, Denmark

Figure 2 Photo of the new lid (10,000 m2) installed on a 75,000 m3 PTES in Marstal, Denmark.

The lid section has several advantages as it addresses and solves water ponding, air accumulation, and thermal expansion issues. 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 leakage detection and seeking are highly improved. Any potential leakage in a section will be led toward 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 increases.

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 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 m2 insulating lid on a 70,000 m3 PTES with a constant temperature of 90°C at the top of the storage.

PTES technology is a mature technology, and it is also used for larger storage. A tender phase is ongoing for a PTES system consisting of 2×500,000 m3 storage 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. The “FLEX_TES” project participants 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.

Meet the author

Morten Vang Bobach
Product Manager, Senior Engineer, Aalborg CSP
“Scaling up pit thermal energy storages” was published in Hot Cool, edition no. 4/2020. You can download the article here:
Scaling up pit thermal energy storages, Hot Cool article, 2020, no. 4