Thread: Sodium sulfate & heat storage

apparently Sodium sulfate is used in heat storage applications as according to lovely wikipedia...........

"The phase change allows a substantial reduction in the mass of the material required for effective heat storage (83 calories per gram stored across the phase change, versus one calorie per gram per degree Celsius using only water), with the further advantage of a consistency of temperature as long as sufficient material in the appropriate phase is available."

So, with this in mind it is obvious that it should really only be chosen if a phase change is going to be incorporated in the design as that is one of its prime advantages (as I also get to use less of the stuff with high heat energy retained in the different phase) also lowering the specific heat capacity of the working fluid therefore needing less heat to initiate a phase change.

However...................is sodium chloride just as good???

secondly - as far as I am aware a substance with a high specific heat capacity may take longer to warm up but equally have a longer warm down period........is this correct (my first assumption)???? if so then i need to raise the specific heat capacity of the storage medium so long as it gets long exposue to the source of heat to get a longer term of storage?????? - is this ture????

many thanks in advance for any help.

Originally Posted by fatman57

apparently Sodium sulfate is used in heat storage applications as according to lovely wikipedia...........

"The phase change allows a substantial reduction in the mass of the material required for effective heat storage (83 calories per gram stored across the phase change, versus one calorie per gram per degree Celsius using only water), with the further advantage of a consistency of temperature as long as sufficient material in the appropriate phase is available."

So, with this in mind it is obvious that it should really only be chosen if a phase change is going to be incorporated in the design as that is one of its prime advantages (as I also get to use less of the stuff with high heat energy retained in the different phase) also lowering the specific heat capacity of the working fluid therefore needing less heat to initiate a phase change.

However...................is sodium chloride just as good???

secondly - as far as I am aware a substance with a high specific heat capacity may take longer to warm up but equally have a longer warm down period........is this correct (my first assumption)???? if so then i need to raise the specific heat capacity of the storage medium so long as it gets long exposue to the source of heat to get a longer term of storage?????? - is this ture????

many thanks in advance for any help.

The melting point of sodium chloride is 801 C. If you are going to use it you must have a very high working temperature in mind.

Heat capacity is not directly related to either the RATE at which materils heat up or cool down. That involves not only heat capacity but also conductivity. If you are talking about something going through a phase change you will probably also have to worry about convection. And if the temperatures are as high as you indicate, radiation will also be a factor.

I suspect that heat transfer is a bit more involved than what you may initially believe.

What are you trying to do ?

If you are going through a phase change (i.e. melting the salt) it is latent heat of fusion that provides the heat storage, rather than specific heat capacity. This is why in the quote it says calories per gram for the salt but calories per gram per degree C for water with no phase change.

Originally Posted by Bunbury

If you are going through a phase change (i.e. melting the salt) it is latent heat of fusion that provides the heat storage, rather than specific heat capacity. This is why in the quote it says calories per gram for the salt but calories per gram per degree C for water with no phase change.

i think the salt is usually placed in a decahydrate state and the phase change comes mostly from the water but am unsure. A phase change of the salt would be quite substantial.

its one technique used with solar energy to store heat. similar principles apply i presume with heat transfer as well - say from the panels themselves to the means of storage.

i appreciate that specific heat capacity does not relate to the rate at which a substance cools or heats but it is a component.

If i were to try and store heat from a solar source it would most likely need to survive the night in a domestic situation, for example.

i guess in this context my question really is concerning the rate at which a material gains or loses heat and whether it is generally found that a substance with a low specific heat capacity would heat easily and cool just as but a material with a high one would show the opposite characteristics (heating slowly and losing it slowly) or if this is to a greater degree relative to the material in question (in lab conditions if that makes this easier)?

or if this is all wrong!!!!................

In a domestic situation why not just use a domestic hot water tank, well insulated to store the heat? Water is a good storage medium because of its high specific heat, it easily pumpable and the domestic systems are already worked out and readily available.

Originally Posted by Bunbury

In a domestic situation why not just use a domestic hot water tank, well insulated to store the heat? Water is a good storage medium because of its high specific heat, it easily pumpable and the domestic systems are already worked out and readily available.

anyone know the rate at which water loses heat?

Rate of heat loss is not a property of water, It is a property of the system in which the water is used. There is no single answer to the question "what is the rate at which water loses heat?"

The capacity of water to store heat (or any fluid) without considering phase chnage is Q = M.Cp. DT

M is the mass, Cp is the specific heat at constant pressure, DT is the change in temperature from low to high as you add heat. The higher the value of Cp the smaller the mass of fluid you need to store a given amount of heat. Water has a higher Cp than, say, hydrocarbons, and a higher density, so relative to hydrocarbons you need a significantly smaller volume to store a given amount of heat.

The rate of heat loss of your stored water will depend on the system it's in. In a simple hot water storage tank the rate of heat loss is heavily dependent on the quality of insulation and only slightly dependent on the properties of water. In a dynamic system with heat exchange to other media in heat exchangers water will give a very high heat transfer rate because of the combination of high Cp and high thermal conductivity, with low viscosity. There is a whole field of engineering devoted to designing systems and equipment to optimize heat transfer. If it was a simple one-liner I wouldn't have a job.