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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I am designing a 10,000 square ft greenhouse (100' x 100'

Customer Question

I am designing a 10,000 square ft greenhouse (100' x 100' x16'). it will be all well insulated - walls R20 and roof R40 - temperature can range from 60 degrees to 80 degrees - i will have thermal storage from heliostats with sand heated to 1,000 degrees. I am inventing method to send storage and heat the sand inside a boiler.
I need to calculate how many heliostats needed
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.


There are several issues here and a bit more engineering required than you may have had in mind.

I will summarize the issues:

- 1,000F heated sand cannot be done the usual hot water boiler. (a high pressure steam boiler will

most likely more complex and costly than you want to get involved with.

- Electric heaters will go to 1,000F but will cost a fortune to buy and operate.

- You do not need to store heat a 1,000F to heat the space to 80F... you can do it with hot water

and a hot water boiler.

- The number and type of heliostats and heat transfer coils is dependent on the outdoor air

temperature variables.

The design done professionally will cost you between $2,500 and $5,000. I can submit an additional services offer for a conceptual design with full parts list and all of the details that a competent contractor would need to install such a system, for $500 if you are interested, That will take me 2 or 3 days, it will come with CAD drawings similar to the one attached below. 3 or 4 drawing sheets.

Let me know what you think, we can go from there.

Expert:  Phil replied 1 year ago.

Regarding the solar radiation, it will work very well with simply hot water collectors in the summer... but will not work at all on many days in the winter when there is no sun. You will need these back up systems I have described above.

Expert:  Phil replied 1 year ago.

The amount of heat per cubic foot that can be stored in water is exponentially higher that can be stored in sand... Getting sand up to 1,000F will require parabolic reflectors and liquid sodium or very high pressure steam vessels, costs will be exorbitant compared to the other options you have.


Customer: replied 1 year ago.
Thanks Phil - I know you need more information to continue, but as long as we can talk a little more, I would like to exchange another thought or two for your consideration.I know the specific heat of sand is about .2 - 10 tons of sand allowed to drop from 1,000 degrees to 500 degrees is a great deal; of storage at a low cost. Water pressure, with a specific heat of 1, above 500 degrees would create a dangerous and expensive pressure. Sand is good at this point from a cost point. Today most storage at this point is phase changing salt, but this is a problem for me - the operators of my system will be simply homeowners or greenhouse operators who are not qualified engineer. Hot sand inside a pressurized storage box can be used to pass on heat energy and making a controlled amount of steam to run a turbine generator, and the waste heat from the generator can be use as cooling.As an engineer, I know you may depend on proven examples, but I have not found any on the internet. One project is Spain comes close. My purpose in talking with you is to find any weak points in my invention. So far I still have confidence in my plan. I have several large contracts for greenhouses that may be willing to fund a prototype.You may be the right engineer to approve, with necessary modifications or details my plan, but of course I need to know if I have persuaded you that you would like to continue. I would also appreciate to know your profile.Thanks very much,Joe Bullen
Expert:  Phil replied 1 year ago.

Hello again Joe,

My background is as a mechanical engineer doing projects for private companies and also for several government agencies.. including the US Dept of Energy...several projects in the multi million dollar range. Nation wide. I will not be blowing smoke at you here.

You do not need any engineers approval... that said no engineer I know would go to the trouble of heating sand to 1,000F in order to heat a green house to 80F ... that would be like building a howitzer that shoots 10 inch shells, at very great expense, and using it to shoot flies.

Fly swatters are already on the market, and a lot cheaper.

Specific heat is not related to how many tons the material is involved... it is the amount of heat required to raise one pound of the given material 1 degree F. . that is one of several confusions that is throwing monkey wrenches into your thought process here... there are several others that have gotten you off on the wrong foot so to speak.

These physics need to be understood, and without that a person can get lost in the tall weeds.

In the context of 'just answer', that level of extensive training is not a fit for our low cost format....If you want to get involved in this sort of work, a junior college course in 'Heat transfer and thermodynamics' would get you off a more solid footing. (the course has some math pre requisites, algebra 1a and 2a)...and it is affordable. Amazon has books on the topic, including the economics of heat transfer also valuable on projects such as what you have in mind.

You are correct about phase changing salt however... the phase change stores about 9 times more heat per cubic foot than water, and 80 times more heat than sand.

All that said, I want to assure you, and you can tell your potential investors that you can indeed use 1,000 F sand as a heat storage medium to heat green houses... but you and they will need to know for sure, that the costs will be 10 times higher than current methods in wide use by growers world wide.

(solar heat to 200F with thermal storage in insulated underground pools is cost effective. co-generation produces power for the grow lights, carbon dioxide for fertilizer, and the waste heat to warm the building).

I will opt out, so that others here can see your question, but I will keep tabs on the progress and you can post to my attention and I will pick the thread up again, but only if you agree to at least rate my first hours work here positively.

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