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District Heating and Cooling

Annex VI Project 05

The Research / IEA DHC Annexes / 1999-2002 / Annex VI / Annex VI Project 05

Optimization Of Cool Thermal Storage And Distribution

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The report "District Cooling, Balancing the Production and Demand in CHP", which was released un-der Annex V, shows that district-heat-driven absorption cooling could be the key factor in improving the chances of combined heat and power (CHP) and making its energy efficiency possible in relatively warm climates. The report also shows that district-heat-driven absorption cooling is not very competitive with electric vapor compression cooling, unless some special conditions occur. This is mainly due to the high investment of the absorption chiller and the cooling tower(s).

Using a chilled water storage in this kind of an application could bring the investment cost down and improve the feasibility essentially. A cool thermal storage decreases the required size of the cooling plant (chiller and cooling tower). This effect is even stronger, if the storage is used for providing a share of the peak cooling demand, so that the chiller plant can be designed for off-peak conditions and the energy efficiency of the cooling process would remain very high.

The basis for the use of a diurnal thermal storage in cooling applications is twofold. Either there is a considerable variation in demand or the variation is in the price of primary energy between day- and night-time. In the former case a storage can be used to reduce the capacity cost of the system. In the latter case using a storage can cut down the operation cost. In most locations the both conditions are true. A thermal storage also improves the reliability and availability of cooling.

Description of the project

In this project now proposed cool thermal storages and (district) cooling distribution systems will be the focus of the research. Information on the cool thermal storage and cooling distribution techniques on the market and under promising development will be collected and analysed. The production technologies of chilled water and ice will also be addressed. The information of existing systems and their parameters will be used as extensively as possible. The analysis concentrates on finding out, which the best production/distribution/storage concepts in different conditions are. The results will provide rules of thumb for the designers to choose between different concepts, if finding such rules is possible.

The research project will present case studies on optimising the sizing and the operation strategy of cool thermal storages, concentrating on commercial concepts. This will help understanding the economic variables and how they fit together. The feasibility and cost-effective scale of distribution will be investigated. The implications of the results on the possibilities of district cooling and its integration with CHP production will be presented


  • Provide information on cool storage, cooling distribution and generation (technology, economics and experience of existing systems)
  • Help understanding the economic variables of (district) cooling applications and their relations
  • Provide information on choosing, sizing and operating a cool thermal storage optimally
  • Provide case study examples with sensitivity analyses
  • Address the implications of cooling system choice on CHP and energy efficiency optimisation, and therefore the environmental impacts

The emphasis in the project will be on commercial cooling techniques, giving up-to-date information of the state of the technology. Promising new solutions will be evaluated according to the information available, and by using the simulation tool for estimating the economic potential of such solutions.

Summary of the final report of the project

This project concentrates on the analysis of suitable cool thermal storage (CTS) solutions for district cooling (DC) systems and on specific technologies, which have wide reference backgrounds of special interest. The report presents different CTS technologies for daily use and shows the selection of feasible and technically most interesting CTS and chiller alternatives for the cooling load profiles of selected DC distribution networks (peak load 30MWcw). Differences between CTS applications and their relations to DC systems are explained by using a selection of operational and economic assumptions. The feasibility of selected CTS alternatives was studied in cases with different loads and operating costs. The two reference cases included fictitious DC systems located in Stockholm, northern Europe (NE) and in Barcelona, southern Europe (SE).

The analysis of different CTS technology on the market and in the developing phase resulted in the selection of chilled water, sodium nitrite/nitrate-water solution, ice-on-coil (external melt), and ice slurry storage for the case study systems. The results of the case studies with sensitivity analyses using three peak/off-peak prices of electricity are presented as graphs, in which the total price of cooling (¢/kWh) is shown as a function of CTS output capacity (%/peak load demand).

In general, ice-based storage is feasible neither of the cases, but the water-based storage system with low investment and operating costs is economically well justified. Typically, the chilled water storage is feasible up to the technical capacity limit of the system (i.e. 30-40%) while the specific investment of storage discharge capacity is lower than the investment of corresponding chilling capacity (when low-cost space is available). Under the assumptions of the SE case ice-based CTS (ice-on-coil) may be feasible if the difference between the high and low electricity tariff is more than 6 ¢/kWh.

With a requirement of 12% (ROE) the overall cooling price levels calculated for SE (from 4.99 ¢/kWh to 5.22 ¢/kWh) are 37% lower than those calculated for NE (from 7.54 ¢/kWh to 8.55 ¢/kWh) despite the fact that the electricity prices are overall lower in NE. This can be explained by the number of cooling degree days, i.e. 40 in NE (Stockholm) and 471 in SE (Barcelona). The calculated prices for cooling should be viewed as indicative (assumptions of no inflation, no tax), and used for relative comparison between the two geographical locations and the various storage technologies analysed.

In addition to direct economic benefits, all the storage technologies shall also be understood as additional recourse for the system operator to increase the flexibility and security of cold production.


Electrowatt-Ekono Oy, Finland


FVB Fjärrvärmebyrån ab, Sweden
Kattner/FVB District Energy Inc., United States