Ice storage system utilizes the night time (off-peak of electricity price) to operate chiller to cool. When the compressor is running and the brine (glycol solution) temperature is lower than 0°C, the water in the tank will generate the phase transformation and then frozen to storage a number of latent heat. Then in the day time (on-peak of electricity price), it will discharge cooling capacity to support the air-conditioning demand and reduce the running cost of chiller. In the meantime, we change the cost of on-peak time into off-peak time, so we not only reduce the power distribution capacity but also we can save electricity cost in using the on-off peak.
Ice storage system divided into the full capacity and component capacity. Take the office building as an example. The air-conditioning load is as figure 1. The on-peak air-conditioning designed load in summer is 1000RT. The air-condition time is from 8 am to 5 pm. The whole day load of air-conditioning is 7500 RTH. The component capacity of ice storage air-conditioning load is as figure 2. The chiller runs continuously. It supports parts of load for air-conditioning while the other part is supported by the ice tank which storage at off-peak time. Because of the long time running, the air-conditioning load will be divided into 24h, and the averaged load is only 312.5RT (31.25% of before data). The ice storage capacity just use to be back up when day time is not enough, so the capacity of ice storage will also decrease. This kind of ice storage mode is the best way to save cost.
The full capacity ice storage air-conditioning load is as figure 3. The chiller storages ice only at the night time (or on-peak time). In the day time, when the air-conditioning works, the chiller will stop to cool. And the load is totally supported by storage capacity. When the end has demand, you can just operate the pump and end fan coil unit. And the averaged load capacity of chiller can decrease till to 500RT (50% of before). Because the load can support by the storage capacity, we need a larger tank, and the investment cost will be more expensive than component capacity. But the full capacity ice storage saves electricity obviously when use in on-peak time. Combined with electricity price of off-peak, the electricity will save a lot.
Choosing component capacity ice storage and full capacity depend on the performance characteristic (24 hour load profile) and the calculation of electricity price. So if you want to choose correctly, you need to evaluate it first. Compared with traditional air-conditioning, the ice storage system can save the power distribution capacity of chiller, water pump and cooling tower and other equipment. But you need to increase the fee of plate type heat exchanger, adjustable valve and ice storage tank. If the cost of ice storage is higher than the equipment capacity that you reduce, you need to rely on the running cost that you save to reach economic benefit. In some conditions, if you utilize building raft foundation as ice storage tank, it just needs to increase the ice storage medium fee. At this time, the investment system fee may lower than traditional air-conditioning system.