HOT|COOL NO.1/2016 "COP21"


By Anders Dyrelund, Senior Market Manager, Ramboll Energy and Flemming Ulbjerg, Chief Consultant, Ramboll Energy

• Step 2: Large thermal storage pits (PTES). The first task has been to store solar heat from large-scale solar heating plants from summer to winter and thereby increasing the solar heat from less than 20% to roughly 50%. The second task will be to utilize interruptible efficient heat sources from CHPs and large heat pumps in order to integrate a larger share of wind energy.

An increasing number of countries are going to implement low carbon climate action plans, with the aim to reduce the fossil fuel consumption in a cost effective way. The challenge will be to integrate the efficient energy sources and the fluctuating renewable energy in the energy system in a cost effective way. Due to economy of scale, district heating can integrate many heat sources, which have sufficient temperature and capacity when needed. The cost of extracting low temperature heat from a large coal fuelled power plant is e.g. less than 0.4 MWh coal per MWh heat in case a similar condensing plant is in operation – not to speak about deep geothermal heat, energy from waste and large-scale solar heat, which can be used cost effectively without fossil fuels. The challenge in the energy system of the future will be to integrate all available renewable sources, which unfortunately are fluctuating and often not available when needed. It is a problem that solar heating and surplus heat from cooling are only available when the heat demand is low. It is an even bigger problem for the power system that fluctuating wind, solar PV and hydro power are difficult to integrate. Pump storages can help in some countries, but cost effective pump storages are also a limited resource. The power sector even considers establishing electricity storages, which are extremely expensive. Therefore, large thermal storages in the district heating will play an important role in the energy system in the future, not only to use all heat sources better, but also to support the power system integrating the fluctuating electricity sources.

The diagram below shows the investment cost in relation to storage capacity in MWh.

Investment costs storage tanks (TTES) and storage pits (PTES) incl. design, construction and materials

Capacity in MWh storage

Figure 1. Cost of thermal storages

This diagram gives us two important pieces of information: • It is important to reduce the temperature at consumer and in the grid to reduce investment costs in storages (plus many other good reasons) • Storage pits is an interesting opportunity to reduce cost and increase volume Pressure less storage tank The largest pressure less hot water storage tank has a volume of 75,000 m3, the maximal temperature is 95 o C and it maintains the pressure in the grid. It can store 4,000 MWh heat and is owned by Fjernvarme Fyn in Odense, Denmark. The second largest in Esbjerg has a volume of 50,000 m3.

Denmark has gained useful experience in this process for several reasons:

• Strong energy policy and tax on fossil fuels • Large share of low temperature district heating <100 o C • Large share of combined heat and power (CHP) • Large share of wind power

The introduction of heat storages has taken place in two main steps:

• Step 1: Heat storage tanks (TTES) to all CHP plants and thereby to almost all 400 district heating systems. The main task was to de-couple heat and power and thereby improving the flexibility and opimizing production of heat and power. Three large tanks are slightly pressurized, all other are pressure less.

Figure 2. Simple connection of a pressure less storage tank

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