Heating & Cooling

Case Studies

Figure 64

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Cooling price with chilled water  storage (7/14°C) and solution storage (1/14°C.Case NE/Stockholm. Base case:Electricity tariff 8 / 5 ¢/kWhe

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In the NE/Stockholm case the cooling price of a system with no storage and with supply/return temperatures of +1/14°C (base case) is 8.02 ¢/kWh. About 60% of this price is due to investment cost repayment, cash flow to equity and interest charges. The remaining 40% of the cooling price consists of variable and fixed operating costs. The cooling price falls when part of the chiller capacity is replaced with sodium nitrite solution storage capacity. With 20% storage the cooling price r educes by 6% to 7.51 ¢/kWh. With 30% storage the cooling price is further diminished to 7.32 ¢/kWh. With 40% storage the cooling price (7.63 ¢/kWh) is 5% lower than in the base case. All the results of the case study with sodium nitrite solution storage are presented in Figure 64 and Table 29. The system with 30% storage results in the lowest cooling price when compared with other cases. The reduction in the cooling price is 9% compared with the base case. About 45% of the total savings consist of savings in operating costs, 55% of savings in investment costs. Owing to a two-time tariff, savings are achieved in operating costs. With 20% (6 MW, 46 MWh) chilled water storage, cooling production on a high load tariff decreases 48% compared with the system with no storage. This leads to annual savings of USD 42000, which represents 44% of total savings. Also, the maintenance and repair costs decrease due to a smaller chiller plant. When the sodium nitrite solution storage system is compared with the chilled water system, it is clear that choosing the lower supply temperature raises the total costs. A base case with a supply temperature of 1°C has 7% higher total costs than the a base case with a supply temperature of  7°C. But if there is lack of space and chilled water storage cannot be applied, low temperature storage can be the solution. The cooling price of the low temperature system with 30% storage is 7.32 ¢/kWh, which is 0.16 ¢/kWh (2%) lower than the cooling price with 7°C supply temperature and no storage. As a conclusion of the above: if low supply temperature is required, the costs are reduced if a storage system is applied.

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