In December 2018, a 15,000 m3 Pit Thermal Energy Storage (PTES) went into operation in Langkazi in the southern part of Tibet. The PTES system enables storing of excess solar heat and helps supply the city with both stable and sustainable heating. By storing excess solar heat produced during summer and releasing it during wintertime, solar heat is able to cover more than 90% of the city’s annual heating needs.


Client:      Arcon-Sunmark Large-Scale Solar Systems Integration Co., Ltd
End user/Owner:        Local government of the autonomous region of Tibet
Location:   Langkazi, Tibet
Status:   Operationel

Seasonal storage integration for optimal use of solar resources

The PTES in Langkazi is a 15,000 m3 water reservoir. The reservoir is lined with plastic linings to retain heat and prevent leakages, while the top of the storage is covered by a lid used to retain the heat and keep the storage tight from rainwater.

The purpose of the PTES in Langkazi is to store excess solar heat from a nearby 22,275 m2 solar heating plant for later release into the district heating network. During summer, the amount of solar heat produced by the solar heating plant exceeds the district heating demand. The excess solar heat is therefore transferred to and stored inside the PTES. The excess heat warms up the water in the storage to approx. 80-90 °C. During winter, when the district heating demand increases and exceeds the solar heat produced, hot water from the top of the storage is withdrawn, while cold water is passed into the bottom of the storage. The hot water from the top is then released into the district heating network supplying the city of Langkazi with both stable and sustainable heating.

This seasonal storage integration allows for optimal use of the solar resources. It boosts the efficiency of the complete energy system and adds flexibility to the heat production. At the time of development, the storage in Langkazi was the first PTES system installed outside Europe.

With a load/discharge capacity of 17.5 MWth, the PTES is able to store 700 MWh heat.

PTES as part of integrated energy system

The project in Langkazi was developed and installed by Arcon-Sunmark Large-Scale Solar Systems Integration Co., Ltd, which was established as a joint-venture between Chinese Jiangsu Sunrain and Danish Arcon-Sunmark. Arcon Sunmark's scope in the project included consultancy and design services. In June 2020, Aalborg CSP acquired all know-how and intellectual properties related to the PTES technology from Arcon-Sunmark.

The 15,000 m3 PTES system was installed alongside and integrated with a 22,275m2 solar heating plant, which is responsible for supplying the storage with excess heat. Combined, the solar district heating plant and PTES cover more than 90% of the district heating plant’s annual heat production. Moreover, a 3MW electrical boiler serves as backup energy supplier during times of low irradiation and is responsible for supplying the remaining 10%.Prior to the project, the majority of the households in Langkazi did not have proper and up-to date heating installations. However, as a result of the completion of this project, the citizens now have access to heat throughout the year.

In addition to supplying stable and sustainable heating to the citizens of Langkazi, the solar plant and PTES annually also reduces the CO2 emission by 3,700 tons. 



Arcon-Sunmark Large-Scale Solar Systems Integration Co., Ltd

Owner: Local government of the autonomous region of Tibet

Capacity of the plant:



Solar peak production: 17.5 MWth
Electrical Boiler: 3 MWth
District heating: 4.3 MWth
PTES capacity: 700 MWh
PTES load/discharge capacity: 17.5 MWth

Annual energy production:     

17,300 MWh district heating
15,570 MWh (90%) from solar/PTES plant
1,730 MWh (10%) from electrical boiler

No. of customers: 1,000
Scope of supply: Turnkey delivery and installation of solar plant, PTES,
electrical boiler, technical installations, technical building,
district heating network and piping.
Expected system lifetime: 25 years
CO2 savings: 3,700 tons/year


Project updates

Click through the pictures to see how the construction progresses in Langkazi, Tibet. (The pictures are organized in chronological order, latest project updates appear first.)