Thermal storage refers to the application of storing thermal energy in materials for later utilisation . Figure below depicts the charge and discharge cycle for thermal storage systems, i.e. the storage of energy (charging) and the use of energy at a later time that benefits the user (discharging).
Thermal storage can result in the reduction of operating costs by producing and storing the energy during periods of low energy supply cost (off-peak/night time) and utilising the stored energy during periods of high energy supply cost (peak/day time).
Thermal energy may be stored in three main ways:
- Sensible Storage
- Latent Storage
- Thermo-Chemical Storage.
In addition, the two common thermal storage strategies employed are:
- 负载均衡策略
- 负荷转移策略
Sensible Heat Storage
Sensible heat storage refers to the heat storage within a medium that does not result in a change of state (e.g. liquid remains liquid or solid remains solid). The two main sources of sensible heat storage applicable to commercial buildings are:
- Water Storage – Due to the high heat capacity of water, tanks are commonly used as the thermal storage medium within chilled water and hot water systems
- Building Mass – By increasing the thermal mass of the building using dense materials (bricks, concrete slabs, etc.) peak loads can be minimised. These dense materials are able to store heat throughout the day, radiating heat back into the space when ambient temperatures have dropped, or alternatively, cool down over night and remove heat from the space during the day.
Latent Heat Storage
Unlike sensible heat storage, latent heat storage utilises a medium that transfers heat by changing state (e.g. liquid to solid). Given this additional phase change capability, latent heat systems have greater capacity to store energy than those of sensible heat storage systems, when the same physical size. The main methods of latent heat storage within the HVAC industry are:
- Ice Storage – Ice is generated and used either directly or indirectly to cool the chilled water system
- Phase Change Material (PCM) – PCM typically use specific salt formulations to increase the freezing point of the material above the chilled water supply temperature, so the material can be frozen with chilled water, to store the energy for later use
- PCM 建筑结构 – 相变材料可用于建筑结构，以增加建筑体的热储存。
考虑到与潜热存储相关的资本成本增加，它们并不像显热存储解决方案那么普遍。 然而，由于潜在解决方案比同等的合理解决方案重量更轻且物理尺寸更小，因此资本成本可能会被结构性节省所抵消。
热化学储存
热化学存储是一种利用介质中可逆化学反应进行热传递的热存储解决方案。 与上面讨论的其他热存储解决方案类似，热化学存储由三个主要阶段组成，如下图所示。
在充电循环期间，热化学材料通过吸热反应（热量输入）吸收热量并产生两种化学产品（化学物质 A + B）。 然后将化学品 A + B 分离并储存。 在放电循环期间，这些化学物质通过放热反应（热量排出）重新组合，从而重整原始的热化学材料并释放热量。
热化学存储的主要优点是它提供的存储容量大约是领先潜在解决方案的六倍。 然而，由于资本成本较高，热化学存储的行业采用率较低。
负载均衡策略
负载均衡策略旨在均衡全天的建筑负载。 这种策略可以通过使用热存储来实现，其中系统在建筑负荷大于制冷机输出时放电，并在建筑负荷小于制冷机输出时充电。 如下图所示，其中制冷机负载恒定为建筑负载的 50%。 负载均衡系统的主要优点是可以减小所需的冷水机组尺寸。
负荷转移策略
负载转移策略是指在非高峰时段对热存储进行充电，以便在高峰负载时可以使用存储的能量。 其目的是将整个高峰负荷转移到非高峰时段，如下图所示。 通过减少或消除冷水机组的运行，可以减少白天消耗的电力。 这种策略通常用于利用较低的非高峰能源成本。
环境与能源部（澳大利亚）
FREQUENTLY ASKED QUESTIONS
Sensible storage involves storing thermal energy by raising or lowering the temperature of a material, typically water or a phase-change material (PCM), within a specific temperature range. Latent storage, on the other hand, involves storing thermal energy by melting or freezing a PCM, which absorbs or releases energy as it changes phase. Latent storage typically offers higher energy density and more consistent temperature output than sensible storage.
Thermo-chemical storage involves the use of chemical reactions to store thermal energy. This method is still in the early stages of development but has the potential to offer high energy density and long-term storage capabilities. In HVAC systems, thermo-chemical storage could be used to store energy generated from renewable sources, such as solar power, and release it as needed to provide heating or cooling.
The load levelling strategy involves storing energy during off-peak hours and releasing it during peak hours to reduce peak demand and flatten the load profile. The load shifting strategy, on the other hand, involves shifting the entire load from one time period to another, typically from peak to off-peak hours. Load shifting can be used to take advantage of time-of-use (TOU) pricing structures, where energy costs vary depending on the time of day.
Water is a common material used for sensible thermal energy storage due to its high specific heat capacity and low cost. Other materials used for sensible storage include phase-change materials (PCMs), such as paraffin wax or salt hydrates, which can store energy by melting or freezing within a specific temperature range. Some PCMs can also be designed to operate within specific temperature ranges, making them suitable for HVAC applications.
Thermal storage systems can be integrated with BMS through the use of sensors, actuators, and control algorithms. The BMS can monitor energy demand, storage capacity, and ambient conditions to optimize charging and discharging cycles. This integration enables the thermal storage system to respond to changes in building load, weather, and energy prices, ensuring maximum efficiency and cost savings.
Thermal storage is commonly used in HVAC systems for air conditioning, heating, and refrigeration applications. It is particularly useful in buildings with high cooling or heating demands, such as data centers, hospitals, and commercial office buildings. Thermal storage can also be used in district cooling systems, where it can help reduce peak demand and improve overall system efficiency.
