Thermal Energy Storage

Thermal Energy Storage

Bridging the gap between energy supply and demand

Storing thermal energy in tanks or in underground installations makes it possible to save excess energy for use at a later point in time – days, hours or even months after. The concept known as Thermal Energy Storage (TES) thereby bridges the gap between energy supply and energy demand.

World energy consumption is projected to increase by 50 % by 2050. At the same time, the world is running dry of traditional energy resources. To accommodate future energy consumption in a sustainable and emission-free way, more and more energy comes from renewable resources, such as wind, wave, and solar power.

But with renewables follow a loss of control – we simply cannot control when or where winds are strong enough to produce power. The lack of control leads to instable production, where energy production might not match energy consumption, resulting in excess energy. There are even examples of wind farms shutting down, although production is efficient, simply because of a mismatch with consumption needs.

What is Thermal Energy Storage (TES)?

To prevent that excess energy is simply left unused and lost, because the time and place of consumption do not match production, a race to find ways to store excess energy has begun. One method is the so-called Thermal Energy Storage (TES), referring to cooling or heating of a storage medium. Examples of storage mediums include liquids such as water or solid materials such as stone.

“Simply put, Thermal Energy Storage makes it possible to store energy during off-peak periods and distribute it during peak periods to utilize energy from renewables when electricity prices increase,” says Allan Jørgensen, Key Account Manager at Senmatic.

“Wind energy is one of the cheapest sources of electricity. When winds are strong during off-peaks and power production surges, it lowers the cost of electricity. Saving the cost-effective energy means that we can utilize it during on-peak consumption, where the cost of electricity is correspondingly high. With Thermal Energy Storage, we can operate based on electricity prices,” tells Chan Nguyen, Special Adviser at Danish district heating company Fjernvarme Fyn and Ph.D. in Mathematical Modeling specialized in energy systems and heat pumps.

Types of Thermal Energy Storage systems

Thermal Energy Storage systems typically include storage in tanks or underground. There are several possibilities for underground storage.

Thermal Energy Storage Tanks:

Thermal Energy Storage tanks work by producing thermal energy (chilled or hot water) and distributing it to the facility during peak periods by warm and chilled water entering and exiting the tank through diffusers at the top and bottom of the tank. The diffuser system is designed to minimize turbulence and allows stratification of the water. During stratification, the colder and denser water moves to the bottom and the warmer and less dense water to the top separated by a thermocline.

The tanks are insulated steel tanks and can be pressurized to match pressure and temperature requirements from the heat transmission system.

Underground Thermal Energy Storage:

Underground Thermal Energy Storage (UTES) systems store energy by pumping heat into an underground space, typically using water as storage medium. In general, large-scale underground systems of more than 4,000-5,000 cubic meters are a cost-effective option, while tanks are the smarter alternative for smaller capacity systems. This does however depend on the price on land, as underground systems require a lot of space.

Underground systems include the following:

Pit Thermal Energy Storage (PTES):

Pit Thermal Energy Storage (PTES) systems are large underground reservoirs lined with plastic lining and covered with an isolating lid. The lid is a vital part of the construction, having to be able to support both rain and snowfall as well as following the movements of the water inside the storage if the temperature changes to ensure stabile and safe operations.

Borehole Thermal Energy Storage (BTES):

Borehole Thermal Energy Storage (BTES) systems are arrays of cylindrical boreholes made in materials such as rock, soil, or clay. They work by transferring heat and cold to the ground material and are designed to seasonally store energy by reversing the flow direction from season to season.

Phasing out fossil fuels

Operating in the energy sector, Fjernvarme Fyn also has ambitions to phase out coal and other fossil fuels to promote the global sustainability agenda. First, the ambition is to transfer from coal to gas and then from gas to fully renewable resources. The latter through Pit Thermal Energy Storage.

Fjernvarme Fyn is one of Denmark’s largest heat suppliers and produces both electricity and power and owns all piping reaching the end consumers. Being the owner of the full supply chain for more than 100,000 housing units, cost effective alternatives are key:

The largest TES facility is not necessarily the better option. Deciding on a TES facility should be based on consumption calculations. Pit Thermal Storage requires a large space, as it is dug into ground. Our initial Pit Thermal Storage facility is 0.7-1 million cubic meters.

Chan Nguyen Special Adviser at Fjernvarme Fyn - and Ph.D. in Mathematical Modeling specialized in energy systems and heat pumps

So, what do sensors have to do with TES?

We now know that temperature is essential for successful TES operations, which is why sensors are essential. By installing sensors, whether in a tank solution or an underground solution, two central temperature-related risks can be monitored: stratification and leakages.

Control stratification

Stratification refers to the division of the water column in the TES installation based on densities caused by differences in temperature. As water enters and exits the installation through diffusers located at the top and the bottom of the tank based on off-peak and on-peak periods, it creates a stir of water.

If, for example, the top layer consisting of the warmer and less dense water is 85°C and the colder and denser water placed at the bottom is 40°C, a middle layer of approximately 60°C is created. To control that the water distributed for consumer use is at the correct temperature, Multispot temperature sensors monitor temperatures throughout the installation. The sensor spots are typically placed vertically every 50 cm in strategic places. If too much water enters the system and the stir causes the layers to mix, sensors will detect the unwanted temperature change, enabling operators to hold back the intake or withdrawing of water.

As tank installations are deeper than underground installations, more water can be added without stirring the middle layer.

Measuring multiple temperatures in different layers or stratification can be done by using Senmatic’s Multi-spot Temperature Sensor type NLI that only requires one flange connection in the tank. Other customized solutions are made in close cooperation with our customer to match the specific application. Alternatively, several thermowells can be mounted on the side of the tank where individual temperature sensors are mounted, such as our type PTE.

Allan Jørgensen Key Account Manager at Senmatic

Detect leakages

Another important use of sensors relates to leakages. Here, sensors are used to control the exterior of the installation:

In underground installations we potentially have to dig out large amounts of soil to find a leak. By installation sensors in a close net structure, for instance placed every 50 cm, we get data revealing thermal loss at a precise location, so we can make our efforts to repair a leak much more effective both in terms of cost and man-hours by digging at the exact location of the leak with a clear idea of the extent of the leak.

Chan Nguyen Special Adviser at Fjernvarme Fyn

In underground installations, external factors such as ground temperature also plays an important part in temperature control.

Investing in sensors by far outweigh the cost of repair that will become necessary without them

Chan Nguyen Special Adviser at Fjernvarme Fyn

Sensors for TES

Senmatic offers several types of sensors for Thermal Energy Storage installations:

  • type NLI – a multi-spot temperature thermometer constructed in a flexible tube to be placed in the medium measuring up to 20 spot positions. If measuring more than 20 temperature spot positions more sensors must be used parallelly.
  • type NS – a wired single spot temperature tank thermometer for measurement in pipeline and tank systems. It has a maximum length of 35 meter and is to be placed in the medium – perfect solution also for specialized tank applications.
  • type PTE – a single spot temperature thermometer with cable. Typically used with thermowell and where access is difficult.
  • type MNS – a mineral insulated single spot temperature thermometer with a length up to 300 meters to be placed in the medium – even cryogenic temperatures – or as leakage detection.

Senmatic has previously delivered type NLI sensors for temperature control of stratification in a thermal heat storage pit in Vojens, Denmark, which was the world’s largest thermal heat storage pit at the time.

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