Sensible Heat Energy Storage

Storage of energy as heat content of matter is inexpensive and easy to implement. It can be applied to space heating and cooling as well as for power generation. Two types of thermal energy are used: sensible thermal energy, essentially proportional to temperature difference, and phase transition thermal energy, such as the latent heat during freezing and melting, which could maintain a fixed temperature with energy content much greater than sensible thermal energy. Phase-change materials (PCM) are well suited for the storage of solar energy.

Sensible heat energy storage utilizes the heat capacity and the change in tempera­ture of the material during the process of charging or discharging — the temperature of the storage material rises when energy is absorbed and drops when energy is with­drawn. One of the most attractive features of sensible heat storage systems is that charging and discharging operations can be expected to be completely reversible for an unlimited number of cycles, that is, over the lifespan of the storage.

In sensible heat energy storage, the thermodynamic process of the material is almost always isobaric, or under constant pressure, typically atmospheric pressure. A solid or liquid is usually used. The specific heat of gas is too low and not practical for thermal energy storage. The heat Q delivered by the material from initial temperature T to

Physics of Solar Energy C. Julian Chen Copyright © 2011 John Wiley & Sons, Inc.

final temperature T2 is

r T2

Q = M cp dT, (12.1)

Jt1

where M is the mass, cp is the isobaric specific heat. In most applications, the density and specific heat can be treated as a constant. Equation 12.1 can be simplified to

Q = Mcp (T2 – Ti). (12.2)

The quantity of material required for the storage tank and the heat losses are approximately proportional to the surface area of the tank. The storage capacity is proportional to the volume of the tank. Larger tanks have a smaller surface area – volume ratio and therefore are less expensive and have less heat losses per unit energy stored.

An important issue in thermal energy storage is thermal conduction, or temper­ature equalization in the medium. In liquids, heat conduction has two major paths: conduction and convection. Temperature in a liquid medium can become equalized much faster than in a solid. Therefore, liquid is preferred whenever applicable. Ta­ble 12.1 shows some thermal properties of commonly used liquids for sensible heat thermal energy storage.

Updated: August 22, 2015 — 3:59 am