As an extension to Nick's salt water idea there would be another one, once again taken from the automobile branches: What about cooling water with a significant admixture of antifreeze (glycol)? In its concentrated version it goes down to about -20 to -30 °C (IIRC). It has slightly less heat capacity than pure water, but offers the low temperature range, low viscosity, thermal stability, corrosion guarding, and a rather cheap price.

Dear Ivica

In our article Combined Solar Heat and Power With Microgrid Storage and Layered Smartgrid Control Towards Supplying Off-Grid Rural Villages

https://www.researchgate.net/publication/263085644_Combined_Solar_Heat_and_Power_With_Microgrid_Storage_and_Layered_Smartgrid_Control_Towards_Supplying_Off-Grid_Rural_Villages

we discuss options for using natural or vegetable oils for energy storage. I would recommend you study the following references also:

Sharma, A., Tyagi, V.V., Chena, C.R. 2009. Review on thermal energy storage with phase change materials and applications. Elsevier Sciencedirect, Renewable and Sustainable Energy Reviews 13 (2009) 318–345.

Suppes, G.J. 2002. Fatty acid thermal storage devices, cycles, and chemicals. US Patent Application US20020011587/US6574971.

Suppes, G.J, Goff, M.J., Lopez, S. 2003. Latent heat characteristics of fatty acid derivatives persuant phase. Chemical Engineering Science, Vol 58, 1751-1763.

Kleinwächter, J. 2004. Solar Power Village: An integral concept to create energy and stable local jobs in southern rural areas. Second World Renewable Energy Forum: Renewing Civilization by Renewable Energy, May 29-31, Bonn, Germany.

Trust this helps

Gerro

Article Combined Solar Heat and Power With Microgrid Storage and Lay...

Ivica

Chapter 3 of the book Agricultural and Biological Sciences Soybean - Bio-Active Compounds is also interesting

http://www.intechopen.com/books/soybean-bio-active-compounds/facilities-for-obtaining-soybean-oil-in-small-plants

Gerro

it depends how long you need thermal storage for few hours any oil can be used. I used used engine oil for thermal storage in solar cooker, you can read my paper "Nahar, N. M. (2003). Performance and testing of a hot box storage solar cooker. Energy Conversion & Management, 44, 1323-1331."

Gerro and Navratna,

thank you very much for your advices. I work with water and PCM as TES mediums, so oil is something new for me, therefore I had to address someone.

I think I will use engine oil. Also, I will take a look publication list you recommended.

Kind regards,

Ivica

It depends on the application, the kind of the storage system (sensible or latent) and the operating temperature of the system. There are many oils used as well as the phase change materials (PCM) such as Paraffin. The following link might be useful:

O. Ercan Ataer, 'STORAGE OF THERMAL ENERGY' Gazi University, Mechanical Engineering Department, Maltepe, 06570 Ankara

http://www.eolss.net/ebooks/sample%20chapters/c08/e3-14-02-00.pdf

also, this article is useful which compares different oils:

Ashmore Mawire et al 2014 'Performance comparison of thermal energy storage oils for solar cookers during charging' Applied Thermal Engineering

http://jr3tv3gd5w.search.serialssolutions.com/?ctx_ver=Z39.88-2004&ctx_enc=info%3Aofi%2Fenc%3AUTF-8&rfr_id=info:sid/summon.serialssolutions.com&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&rft.genre=article&rft.atitle=Performance+comparison+of+thermal+energy+storage+oils+for+solar+cookers+during+charging&rft.jtitle=Applied+Thermal+Engineering&rft.au=Mawire%2C+Ashmore&rft.au=Phori%2C+Abigail&rft.au=Taole%2C+Simeon&rft.date=2014-12-01&rft.issn=1359-4311&rft.volume=73&rft.issue=1&rft.spage=1323&rft.epage=1331&rft_id=info:doi/10.1016%2Fj.applthermaleng.2014.08.032&rft.externalDBID=n%2Fa&rft.externalDocID=10_1016_j_applthermaleng_2014_08_032¶mdict=en-US

Envisioning the very nice heat capacity values of water, the basic simplicity of its handling, and the accordingly low required volume for a heat storage vessel I'd like to point to the alternative of changing the design of the heat exchanger itself (use metallic fins and the like to enlarge surfaces to increase heat conduction through solid phases into the still liquid portions of the medium).

W/r/to oil please take into consideration that it becomes rather viscous at low temperatures, so convection will be strongly hindered and you end up with (almost) pure heat conduction at the low temperatures if you do not want to introduce an energy consuming convection pump into the system. Due to the high viscosity of the medium such a pump would probably consume quite a lot of energy to be sufficiently effective from a fluid dynamic point of view.

Dr Roosen,

I agree about the water, but I get ice on the tubes. Also, when I run tests with paraffin RT21 I get solidification. I know I need to change heat exchanger but before I do that I would like to run 3 experiments using oil.

I attached temperature of the heat exchanger when performed with RT21 as storage medium.

Kind regards,

Ivica

Ivica,

of course you will get solidification in that temperature range. Therefore my suggestion to modify the internal surfaces, like introducing fins to the copper tubing to distribute the heat more evenly into the space. It may help to put a small pump into the container in order to keep the medium moving. I don't think you will need a large one, but please make sure the pump stops before freezing itself! :)

If you want to experiment with oil I'd suggest engine oil for winter time: 0w20 or 0w30. It will stay liquid, have a (comparably) low viscosity relative to other oil sorts, and the heat capacity will be quite similar to other oils. Take the cheapest possible variants and, if possible, take a look at specified viscosities. Take the lowest possible ones.

Cheers,

Petra

This informaton may be useful for your research

Thermal oil, Santotherm 55 can be effectively used as it has good thermal properties. 

Density- 886 kg/m^3; specific thermal capacity= 1907 J/kg-K;                           Temperature range = -30 to 300 degree celcius,storage density= 59 MJ/m^3;      Thermal conductivity=0.1891 W/m-K; Dynamic visocity= 0.0105 Pa s; Prandlt No=160

However, ionic liquid C8mimPF6 are better than thermal oil. For reference

Ref: www.nrel.gov/csp/troughnet/pdfs/rreddy_ionicfluids.pdf

Have you had a look at salty water ? For the range you mention it could be a solution since it stays liquid down to -22°C and it has the highest Cp (thus lowest volume for same amount of heat). Of course a corrosion problem could appear if it is a long range accumulator. However it is worth while to have a look.

It could be of interest to know what you want to do since the temp range is quite narrow.

As an extension to Nick's salt water idea there would be another one, once again taken from the automobile branches: What about cooling water with a significant admixture of antifreeze (glycol)? In its concentrated version it goes down to about -20 to -30 °C (IIRC). It has slightly less heat capacity than pure water, but offers the low temperature range, low viscosity, thermal stability, corrosion guarding, and a rather cheap price.

I fully agree with Petra's suggestion. I mentioned water because of its high heat capacity (smaller mass for same amount thus lower cost for the container and its insulation), its low viscosity (lower energy demand for circulatoion) and of course its VERY low price. There is an other reason, we always are tempted by "high level" solutions and the basic idea should be " keep it simple".

thanks Nick and Petra, you gave very nice suggestion which I am going to apply.

Petra has given some exceptionally valuable suggestions here. However, I would tread carefully about using fins in water/liquids.

why is that prof Raghhavan? finned tubes are excellent for convective heat transfer in water storage. Here are the one I am using.

Dr. Ivica, what you show are low-finned enhanced tubes which undoubtedly augment convective heat transfer. These are most useful in natural convection and in laminar forced convection. Because of their very low finned height, the conduction path length through the fin is quite small. Under such conditions, yes finned tubes work well.

I was referring to Petra's statement that CONDUCTION can be improved by adding fins to the surface (“use metallic fins and the like to enlarge surfaces to increase heat conduction….”).

Forgive me for taking you to basic fin theory. While fins offer a larger area for convection thus lowering convection resistance, they also increase the resistance to heat conduction. If the reduction in convection resistance is more than increase in conduction resistance, then everything is all right. For this to happen, it requires that the heat transfer coefficient be low, the conductivity of the fin be high and the fins be short.

This happens best with gases in natural convection with copper or aluminium fins, as seen in heat sinks for electronic applications. The other extreme condition would be long fins, turbulent forced convection in water and stainless steel fins.

I am referring here to single phase convection in non-corrosive environments.