Sensible Heat

Storage as sensible heat

Sensible heat

Sensible heat can be detected by our senses as temperature. It is measured as heat capacity. For example water needs 1 calorie - 4.2 joules to heat a gram one degree C. That is 4.2 KJ/KG. To heat it from 0-100 oC will take 420 KJ (0.42 MJ). Water has the highest heat capacity of any substance. But it can only be heated 100 deg at atmospheric pressure. Graphite has a far lower heat capacity, but can be heated to 1800 degrees C.

Latent heat is hidden and we cannot detect it with our senses. It is the energy needed to melt, or vaporise a substance. The energy is required to break the bonds in a crystal, (melting), or to energise molecules to break free of the surface of the liquid.

Chemical energy is similar but atoms are held in place by stronger chemical bonds. (Hydrogen, ionic, or covalent) Sometimes heat can break and reform chemical bonds. Then, when the reaction reverses, heat is released. For example when ammonia is broken apart, it takes in heat. Then when the N2 and H2 recombine to form ammonia, heat is given off.

Definition: Wikipedia

Heat capacity Wiki

Enthalpy of vaporization Wiki

 Sensible heat can be sensed.

                                                  Latent heat is hidden from the senses.

kJ/kG Heat capacity at 25 deg. Heat of fusion Heat of vaporisation Heat of formation
Water 4.2 334 2,260  
Salt - mixture 4.5      
Air 1 203    
Ammonia 4.7 339 1,390 2,700

 

Sensible heat - Molten salt

The challenge is to store the heat at as high a temperature as possible.

Heat from a concentrating solar tower is usually stored  in molten salt. It can then produce steam and generate power during the night or in cloudy weather. 

Overnight about 1% of the heat is lost. The round trip efficiency is about 93%.

The salt normally used are a mixture of NaNO3-NaNO2-KNO. New mixtures are being developed. The main aims are to lower the melting point and raise the temperature the salts can reach before dissociating. At present this is around 700 deg C max.

University of Alabama

http://ras.material.tohoku.ac.jp/~molten/molten_ps_query1.phpMolten salt database

 

Salt mixtures for heat storage

Salt system mixture MP Heat capacity KJ/KG
Solar Salt NaNO3-KNO3 222 1.5
Hi-Tech salt NaNO3-NaNO2-KNO3 142 1.4
U of A salt LiNO3-NaNO3-KNO3 116 2.3
       

Sensible heat - Hot concrete

University of Arkansas researchers use parallel plates of concrete with steel pipes running through them. The steel pipes transfer heat into the concrete.

Concrete can take temperatures up to 600 degrees Celsius and absorbs heat at an efficiency of 93.9%.

        Lead-acid batteries cost f $25 per kWh,

        Lithium-ion batteries cost $50 to $100 per kWh.

        The cost of heating concrete is $0.78 per kWh. However to this must be added  the cost of converting it to electricity.

Ref Science Daily

 

Sensible heat - Hot graphite

On King Island Tasmania the graphite blocks will be heated with electricity from the wind turbines to a temperature of 800°C. The energy can then be recovered later via a steam turbine.

There are also solar thermal projects at Lake Cargellico in western NSW.

This technology is being developed by Lloyd Energy an Australian company. 

The beauty of purified graphite is that the heat capacity increases with temperature. It  can store 3.6MJ/KG (1,000 KWH/tonne) at 1800 deg C.

 Ref.

 

 

Isentropic pumped heat electricity storage

"Isentropic has designed a system that uses the Isentropic heat pump to store electricity in thermal form ("Pumped Heat"). The storage comprises two large containers of gravel, one hot (500C) and one cold (-150C). Electrical power is input to the machine, which compresses/expands air to (+500C) on the hot side and (-150C) on the cold side. The air is passed through the two piles of gravel where it gives up its heat/cold to the gravel. In order to regenerate the electricity, the cycle is simply reversed. The temperature difference is used to run theIsentropic machine as a heat engine.

The round trip efficiency is over 72% - 80%. Because gravel is such a cheap and readily available material, the cost per kWh can be kept very low - $55/kWh - and $10/kWh at scale."

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