Thermal storage in desert environments.

Hello all, I am new to the forum and interested in the upcoming PDC. I have a question regarding measuring thermal storage of a house. I am in the planning stages of creating a food forest and home and shop in South Phoenix.

7 years ago I was staying in the Phoenix Earthship in Taos. At 11am on a June morning I stopped for ten minutes to marvel at the airflow through the doors of the greenhouse. This Earthship has a large greenhouse with a large central opening skylight; the convection for the solar gain was impressive to experience/feel. The observation led me to thinking about using a greenhouse as a heat pump for a home in the desert.

My current plans involve a large basement under the structures, with air tubes feeding ventilation from the north side. The earth is 72 degrees at 5 feet here. The air path to ventilating the greenhouse will turn the structure into a double pass radiator of sorts for the cooled/circulating air. I also intend to reuse the excavation to make CEB walls, and wish to attempt to create a Rasta like brick/panel to insulate the outer shell. The roof will be a variation of a insulated cool roof using radiant barrier under a metal deck.

My question is how do I factor the thermal storage/temperature change of the basement, walls and floor of the structure, and calculate the r values needed on the walls  to insulate for 100 degree evenings and 115-118 degree days for weeks at a time? I believe it takes about 3 months of lag time  for the ground temperature to come up or down in a non insulated berm. Curious to run the math, and gather inputs on factors I might not have seen.

Water is easier. If you had a mass of water, in tanks or a pool or whatever, in the greenhouse, it would heat during the day and give it off at night. Manipulating the air circulation would allow you to control how much the water is exposed to air, thus how much heat the water gains.

If it's too cool at night, redirect some airflow over the water during the day. If it holds too much warmth into the morning, then less airflow is necessary. If using above-ground tanks or barrels, all that would be needed is a louvered vent, or a window or door that could be opened to different sizes, in the vent's path, and maybe a thermometre that you watch, or that sets off a little alarm when the water temperature reaches a level appropriate for nighttime temperatures. It could even be automated to close the vent/door/window when the temperature is reached.

Of course, I am looking at it backwards. Have you considered having a large cistern in the path of the cooling tubes? You need to keep drinking water somewhere. It could act as a huge thermal reservoir, but in reverse, to keep your structure cool.

What detail you've given, though, sounds like you have a fair grasp on what's needed. You basically need to know how much mass you have, and how much heat that loses per hour at night, or how much gain you get during the day versus how cool you can make the basement at night.

But let us know how it goes, and good luck.

-CK

Fergie Ferguson wrote: Curious to run the math, and gather inputs on factors I might not have seen.

Hi Fergie,
in Tucson, AZ there is a research center for cooling near the airport. They have collected a great deal of data and created math models. One of their tested methods is a crawl space or basement of 4 to 6" rock thru which air from the house is forced thru during the day. At night, the rocks were allowed to expel their heat to the outside via fans.

I am not a fan of using electric fans; my belief is that convection can be used in all cases. There are other things I wasn't impressed with but what I was impressed with is their data and how they created math models for these systems.

If you cannot find this research center, I will ask a friend of mine who was an employee there in the 90's.

***Update***
The University of Arizona ENVIRONMENTAL RESEARCH LABORATORY.
2601 E. Airport Drive Tucson, AZ 85756. Tel: (520) 626-3322.
https://ag.arizona.edu/SWES/erl/

Also, for every material you have touching another material, here is the equation (I hope I am not patronizing you with this eq)

   Q = m cp dt                                   (1)
   where

   Q = quantity of energy transferred (kJ, Btu)

   m = mass of substance (kg, lb)

   cp = specific heat of the substance (kJ/kgoC, kJ/kgoK, Btu/lb oF)

   dt = temperature difference (rise or fall) in the substance (oC, K, oF)

If you'll notice, every thing is driven by the temperature difference, everything else is a constant you can look up.

My question to others reading this post: do you know of an equation with takes time into account: maybe a double differential over time and over changing temperature???

Fergie Ferguson wrote:
My question is how do I factor the thermal storage/temperature change of the basement, walls and floor of the structure, and calculate the r values needed on the walls  to insulate for 100 degree evenings and 115-118 degree days for weeks at a time? I believe it takes about 3 months of lag time  for the ground temperature to come up or down in a non insulated berm. Curious to run the math, and gather inputs on factors I might not have seen.

My first response was airheaded as usual. Here is a more thoughtful one:

There are three methods by which heat energy is gained or lost:
I. -conduction (which is the first equation i provided)
   Conduction occurs when two materials are touching each other across the surface area which they touch

II -convection (my dear dear friend, removes the need for fans which fail and eat electricity)
   convection  occurs when a fluid or gas touches a solid material over an area large enough to create a flow in the gas or fluid. The flow is created by a density change with in the fluid or gas usually over atleast an 18" height difference. Sounds like you have both air and water to consider here.

III -radiation  (also dear to me)
all bodies give off radiation heat unless they are at 0 degrees kelvin. this is a more esoteric subject which more elegant minds than mine can explain without delving into browning movement and randomness of where particles are. just find out what the variables stand for and plug and chug if you don't want to open that very interesting box.

In my search for you, I just discovered this pdf (I'll try and attach it).You'll find there are many mathematical ways to express the three methods of heat energy transfer; so don't be alarmed at the lingo of the equation.

For each surface of material, you will need to calculate each of these (when each is relevant).  

What I have found for my own calculations is a bit of frustration because I know how to find the specific heat of fir but how do I find the specific heat of my species of fir?  What is the specific heat of my particular type of dirt?  This is why I referred you to the Lab in AZ, they have methods you might want to consider.