A. Di Cecca, F. Benassis, P. Poeuf
Climespace – GDF Suez, Paris
Thermal energy storage is an important contribution to the rational energy use and allows reducing the environmental footprint helping to comply with environmental constraints. Decoupling the energy use from the supply, cool storage systems integrated in district cooling allows significant reduction in installed cooling capacity.
The energy storage together with an optimized management for cooling buildings also allows the use of electrical energy with the lowest carbon content during the night and at the lowest costs.
The “central” district cooling of the city of Paris includes today 6 cross linked cool generation plants with a total cooling capacity of 215 MW, with an additional 140 MWh/day cooling generation capacity from different storage units installed on three sites. The cool storage systems coupled to the district cooling network in Paris optimise the plants operation and allow for more flexibility.
The growing problems of fossil fuels availability and consequently the increasing energy prices and the environmental restrictions need the energy sector not only to develop new technologies and improving existing ones but also to use energy more rationally. Many intentional agreements have been signed and Governments committed to challenging objectives of energy consumption and CO2 emissions reduction as well as energy efficiency improvement. Thermal energy storage is an important contribution to the rational energy use and allows reducing the environmental footprint helping to comply with environmental constraints. Separating the production from the demand and shifting and attenuating peak loads, thermal storage systems contribute to optimise the advantages inherent to high capacity storage of sensible and latent heat.
Advantages of cool storage systems in district cooling
Considering the cycle efficiency of different energy management types for cool storage, it can be demonstrated that the most efficient system for cooling buildings is the storage created off-peak and stored at the site, and not on the grid. The benefits of energy storage at site have been proven in the HVAC sector.
The traditional air-conditioning in commercial buildings works during the day and are generally off during the night. The chillers are selected to meet the maximum theoretical energy demand during the hottest days of the year. Those systems thus operate mostly below their nominal power with reduced performances.
Decoupling the energy use from the supply, cool storage systems integrated in district cooling allows significant reduction in installed cooling capacity (from 15% to 50%) and all auxiliary components. Cool storage systems avoid compressors running at part load, which decreases the system performances; moreover compressors and transformers capacity can be reduced as well as the electrical power subscription.
The cooling energy available from storage units during the day avoids the installation of additional chillers, which reduces in particular the use of refrigerant whose “Total Equivalent Warming Impact”, albeit reduced in a district cooling system, still contributes to the global warming.
The cool storage systems help not only to reduce the installed cooling power, but also the refrigeration system capacity and size for air-cooled or water-cooled chillers. Consequently, the limited capacity and size of refrigeration towers or dry coolers can significantly reduce the environmental impact (noise and local warming). In fact, the effect of “thermal island”, i.e. the local warming effect generated by the heat transfer from air conditioning systems to the surrounding is thus attenuated. The energy storage together with an optimized management for cooling buildings also allows the use of electrical energy with the lowest carbon content during the night and at the lowest costs. In France, for example, the electricity used during the night for cool storage systems is mostly from nuclear and has therefore low carbon content. Cool storage systems created off-peak contributes to attenuate the peaks for electricity from the grid during the day, thus reducing the size of the electricity transport infrastructure and the associated investments for extension. In addition, since cooling generation is shifted with respect to customers’ cooling needs, storage systems allow cooling without running the chillers when the energy price is the highest (if different electricity prices exist according to the consumption period).
In particular, the latent heat storage integrated into a refrigeration plant provides an effective solution for investors seeking to optimize their equipments and maximize the energy and environmental performances, because it can simultaneously reduce the installed cooling capacities, the amount of refrigerant used in the plant and the operation costs (lower electricity prices), improve the cold production reliability thanks to greater operating flexibility, increase the energy density and the transportation capacity.
Moreover, cool storage systems represent a strategic interest securing cooling generation and distribution.
The sensible heat storage, due to a usually high volume of water available, helps to secure the district cooling network:
– It represents a large volume of water when filling the network, capable of maintaining or quickly restoring cold deliveries to the costumers.
– It allows delivering a significant cooling capacity, in order to strengthen the continuity of cooling supply.
In high density cities, the most efficient way to meet increasing cooling demand, without causing huge picks for energy from the grid, is to develop district cooling with thermal storage.
Investment savings due to the smaller size of the machines used for the ice storage system often exceeds the installation costs of the storage system itself. Those costs include not only the tank cost, but also the necessary surface for the installation, the thermal insulation and the pumping system.
Cool storage systems in the Parisian district cooling
District cooling networks and refrigeration plants developed in an urban environment can benefit of the advantages inherent to the large sensible and latent cool storage systems. It is the case of the district cooling network in Paris, where the cooling needs are higher than the cooling generation capacity installed. The cool storage type is chosen according to the objectives and constrains of the cooling distribution network. The storage systems in Paris are working on a daily basis with a daily cycle “charge/discharge”. The main technical parameters are shown in table 1.
Storing during the night the surplus energy to be distributed at peak hours during the day represents the additional capacity available at lower cost than that obtained by chillers production (comparing the storage cost to the capacity used to generate it, particularly in a liberalised electricity market context). Storage usually occurs at night simply because the lower ambient temperatures make the creation of the cooling more efficient. The strategy is independent from the storage type, latent or sensible storage:
• storing during the night allows benefiting of lower electricity prices and lower outside temperatures, with the reduction of the condensing temperatures and the improvement of the refrigeration units performances, for the plants using refrigerating towers.
• storing during electricity off-peak hours allows reducing electricity costs and the CO2 content of the electricity used for cooling generation. We calculated CO2 emissions economy of 200 kg CO2 for one day using cool storage systems(based on 140 MWh of cooling produced by the storage units).
Moreover, cooling production issued by storage systems avoids the noises often associated to the chillers.
The cool storage systems coupled to the district cooling network in Paris optimise the plants operation and allow for more flexibility, instead of being driven only by the demand. Programmed maintenance works and critical situations due to unexpected production or distribution disruption can be managed with lower impact on the continuity of the service and then on the customers. In case of an emergency, the response of storage systems can be rapid (from 10 to 20 minutes according to the storage type, as shown in table 1), and in any case quicker than that of conventional chillers.
In this way, the investment for the equipments dedicated to thermal storage can become easily profitable: the machines run as long as possible at their nominal rates, improving the Energy Efficiency Ratio (EER) and their behaviour.
The “central” district cooling of the city of Paris includes today 6 cross linked cool generation plants with a total cooling capacity of 215 MW, with an additional 140 MWh/day cooling generation capacity from different storage units installed on three sites (table 1).
The first cool storage system was installed in the basement of the “Les Halles” plant. It is a sensible storage system, with a volume of about 13 000 m3; the tank is divided into 13 equal compartments, one being always empty. The volume losses of this storage system are then 1/13=7.5%. During the charge, the empty volume is filled by cold water while the compartment containing warmer water is drained on the network return. During the discharge, the system is inverted.
The storage capacity is equivalent to a nominal cooling capacity up to 17 MW (table 1), according to the return temperature of the water stored (usually between 2°C and 5°C). The energy available is about 90 MWh. In order to guarantee the cooling supply at peak times and to level off electricity demand, two ice storage units (internal and external melting), with an overall cooling capacity of about 20 MWh (5 MW of chilled water at 1°C during 4 hours) were installed at the refrigerating plant “Opera” at the beginning of 2000 (table1). Those units are located at the basement of a mall in the city centre. The objective was double: to increase the plant capacity during peak times (because of the very high demand concentration in this area) and to postpone the installation of a new plant.
A further four ice storage units (internal and external melting) with an overall cooling capacity of about 30 MWh were installed in the refrigeration plant “Les Halles” in 2006 (table 1). This plant is equipped by ten chillers representing a total mechanical cooling capacity of 44 MW; the ice storage units supply an additional 5 to 13 MW cooling capacity to the city cooling network. Two centrifugal chillers of 3.8 MW each are used for the ice storage. The main advantages of this storage system is to decrease the network cold water temperature from 4°C to 2,2°C in order to increase the density of the energy transported by the existing network and, at the same time, increase the cooling distribution capacity of the plant, without adding generation capacity. The integration of the ice storage units into the hydraulic network of the cold water generator is achieved by two flat plate heat exchangers. The ice storage units are hybrid units with a very high discharge output within a short time at very low cold water temperatures. The high discharge rates result from the simultaneous discharge via the glycol circuit and the water circuit of the ice storage units. An air injection system integrated in the ice storage units generates an additional increase in output and guarantees an optimal constant melting and flow-through of the ice block. An essential advantage of this ice storage type is the optional part-melting and part-loading without affecting the function or the efficiency of the ice storage units.
Fig. 1 : Summer operation for one day in July 2009
The ice storage units in the Paris’s district cooling network are mainly used to reduce the network temperatures from 4 to 2 °C and thus transport a considerably higher load through the existing system at the maximum flow rate. The ice storage units also serve as stand-by in case of the chillers fail and also as reserve at peak loads occurring mainly in the late afternoon. The integration of cool storage units into a power management system in order to reduce the electrical power requirements and avoid peak loads has an interest in a liberalised electricity market context, reducing the impact of the electricity price spikes on the cooling production cost. In order to make the district cooling network more reliable, various ways of strategic operations can be identified:
• chillers operate in COP-optimised part-load operation, storage units fulfilling load peaks. The cooling of the return water in the network, pre-cooled by the Seine river, is completed by the storage systems – inter-seasonal period
• storage units are fully loaded during the nights and weekends, even if the utilisation of the district cooling network on Sundays amounts to only 60%. To compensate high load peaks at the beginning of the week, the district cooling network is cooled down to 2°C incorporating the hybrid ice storage units. The network itself thus becomes a cooling storage unit (8000 m3 of water) – hot summer period
• chillers are meeting the cooling demand as long as possible; the storage units are set into operation only if demand for that specific day are requiring that capacity. The loaded storage units are thus taking over an emergency cooling function if one or more chillers should fall. – normal summer period.
• the free-cooling can be used to store the energy that will be used during the day – winter period
The curves on figures 1 and 3 show an example of a summer day operation and a winter day operation in 2009. Up to 17% of the cooling demand during peak hours (occurring late in the afternoon) can be met by the storage systems in the summer day considered, which translates into a maximum of 15% of electricity peak reduction with respect to the system requirements without storage (fig. 2).
Fig. 2: Electricity consumption corresponding to Fig. 1
Electricity tariffs are higher in winter than in summer. According to the regulated tariff, the highest electricity tariffs are occurring two hours in the morning and two hours in the late afternoon (fig. 3). Cool storage systems are then used to reduce electricity consumption during peak hours in order to reduce costs. In the example, chillers can displace their production from peak to off-peak hours and reduce their production up to 20% during peak times.
Fig. 3: Winter operation for one day in January 2009 Feedbacks and perspectives
Figure 4 is showing the 2009 figures for the “central” network. Cool storage, with a share of 10% in total cooling capacity, is used to attenuate the peaks and to reduce the temperature of the water circulating into the network in order to increase transport capacity (around 10% for the Les Halles plant). But the district cooling system in Paris cannot completely benefit from storing the energy during the night, since about 90% of the stored energy is generated by chillers refrigerated by the Seine river water: in summer, the condenser temperature in the night is only slightly lower than the condenser temperature during the day. So the improvement of performances is limited.
Fig. 4: Extract of the load duration curve in 2009
Sensible cool storage system (LTM, table 1) is the more flexible system with a cycle efficiency ranging from 75% to 85%. But the volume required for sensible storage systems is higher than a direct cooling generation system (1.7 m3/kW vs 0.9 m3/kW). For ice storage (Les Halles and Opera), despite little maintenance requirements, the chillers operation remains complex but the overall cycle efficiency is slightly higher.
Considering the cooling needs, it will be important to develop the storage capacity not only to attenuate peaks but also to optimise the cooling energy distribution. The reliable cooling supply in the extreme summers of 2003 and 2006 convinced many sceptics of the importance of the district cooling combined with cool storage. We estimate that the city of Paris will require additional cooling capacity of about 100 to 150 MW in the next 5 year and it would be important to increase the share of storage in cooling capacity. We estimate that a 40% storage share in total cooling capacity is a reasonable objective that can be achieved.
Inter-seasonal storage can be also developed thanks to the low Seine river water temperature during the winter season (less than 5°C). The water stored in underground tanks during winter represents a cooling capacity to be used during the summer period.
The City of Paris as well as other major cities (Barcelona, Lisbon, London, Stockholm, Helsinki, Copenhagen, etc.) have realised that district cooling is the more effective way to face the present and future energy and environmental challenges for building cooling in high-density cities. Most recently, because of the increased need for comfort cooling, the highest electricity peaks in many regions and countries occur in summer, often with the risk for outages. Cool storage systems optimised integration into district cooling network will contribute to reduce investment for further peak electricity capacity and to strengthen electricity infrastructures.
Due to the high electrical load peaks in the summer period – and simultaneously reduced capacity at the power stations caused by the shortage of cooling water – the time shifted refrigeration at low tariff periods will become more important in the next future. Thanks to the contribution of ice-storage units, cooling peaks can be met and at the same time the highest prices for electricity avoided.
Cool storage systems avoid compressors running at part load, which decreases the system performances. A significant increase of the system efficiency can be reached if the storage systems are running under favourable external conditions, preferably during the night (outside temperatures are lower and thus also the re-cooling temperatures of the chillers) and not when temperatures are higher during the day.
A reduction from 15% to 50% on the installed power can be reached, so the costs are considerably reduced and the overinvestment necessary for the thermal storage system can be compensated by the savings realised.
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