Open Source Climate Battery Design

A climate battery is a low-tech heating system comprised of a series of underground tubes to transfer heat beneath the floor of a structure, where it is “stored” for later use. This page is an open source hub for how to construct one. It discusses climate batteries with the following sections:

• What We’ll grow (click for complete page)
• What is a Climate Battery?
  • Examples of Working Climate Batteries
• Why Open Source a Climate Battery?
• Ways to Contribute
• Consultants on these Designs
• Climate Battery Design Elements/Considerations
  • Air Flow Rate
  • Climate Battery Tubing
  • Soil
  • Insulation
  • Fan Selection
  • Controls, Sensors, and Automation
  • Heating and Ventilation
  • Solar Radiation Heat Gain Considerations
• Validation
  • 2D Modeling of the Climate Battery
  • Evaluation of Boundary Conditions
  • Simulation
  • 3D Modeling of the Climate Battery
• One Community’s Climate Battery Design
  • Tubing
  • Burial Depth
  • Controls, Sensors, and Automation
  • One Community’s Climate Battery Building Plan
• Maximally-affordable Walipini Greenhouse Design Elements and Plans
  • Greenhouse Design Elements: Heat, Loss, Gain, Insulation, Humidity, Sunlight, Cost, Wind
• Resources
• Summary
• FAQ

 

NOTE: THIS PAGE IS NOT CONSIDERED BY US TO BE A COMPLETE AND USABLE TUTORIAL UNTIL WE HAVE OUR CLIMATE BATTERIES COMPLETED AND TESTED AS PART OF THE CONSTRUCTION OF OUR WALIPINI DESIGNS.
AT THAT TIME WE WILL ADD HERE THE COMPLETED PLANS AND EVERYTHING ELSE WE LEARNED.
IN THE MEANTIME, WE WELCOME YOUR INPUT AND FEEDBACK

 

RELATED PAGES (Click icons for complete pages)

 

WHAT IS A CLIMATE BATTERY?

A climate battery is a heating and cooling system that uses forced air and low-temperature geothermal energy. Also known as a “heat bank” and/or “thermal banking”, it works by circulating air through rows of underground tubes where the earth temperature regulates the humidity and temperature of the air, reducing the need for external heating or cooling. It’s called a climate “battery” because of the system’s capability to store heat from the incoming hot moist air and expend it when needed.

 

EXAMPLES OF WORKING CLIMATE BATTERIES

Here’s a 2-minutes video of climate battery maintained passive greenhouse in Nebraska:

Here is a 20-minute more detailed version and a PDF download with additional details:

Here’s another walipini example from Colorado. It includes a few other sustainable heating options too:

 

WHY OPEN SOURCE A CLIMATE BATTERY?

One Community began researching climate batteries for use as part of our Aquapini/Walipinis and Tropical Atrium. We are are open sourcing the research and our designs to encourage others to build their own climate battery. An open source model also allows for discussion, experimentation and innovation which will lead to better systems and ultimately provide a more reliable, efficient, and sustainable heating and cooling solution for greenhouses, homes, and non-residential buildings alike.

 

WAYS TO CONTRIBUTE TO EVOLVING THIS SUSTAINABILITY COMPONENT WITH US

SUGGESTIONS | CONSULTING | MEMBERSHIP | OTHER OPTIONS

CLICK HERE TO HELP US FINISH THE OPEN SOURCE DESIGNS FOR THESE STRUCTURES

 

CONSULTANTS ON THESE OPEN SOURCE DESIGNS THAT ARE COMING

 

CLIMATE BATTERY DESIGN ELEMENTS/CONSIDERATIONS

The following sections discuss the design elements and considerations for climate battery implementation. In order to build an efficient system that takes full advantage of your climate battery’s heating and cooling potential, you will want to pay close attention to these details and designs accordingly.

• Air Flow Rate
• Climate Battery Tubing
  • Materials Considerations
  • Total Length Considerations
  • Depth Considerations
  • The Temperature Variation of the Soil
  • The Water Table
  • Increased Resources to Install Your System
  • Tubing/piping Diameter Considerations
  • Tubing/piping Space Considerations
• Soil
  • Soil Temperature
  • Soil Thermal Properties
• Insulation
• Fan Selection
  • Fan Type
  • Total Air Flow
  • Static Pressure
• Controls, Sensors, and Automation
• Heating and Ventilation
  • Designing for Heating
  • Designing for Ventilation
• Solar Radiation Heat Gain Considerations

 

AIR FLOW RATE

The air flow rate within the climate battery will impact the heat transfer and the overall efficiency. Unfortunately, there has not been enough research around the optimal air flow rate to offer a specific recommendation. A study of Earth-air heat exchanger systems found that increasing the air velocity decreases the thermal performance of the system. At higher air velocity the air does not have enough time within the pipes to reach thermal equilibrium with the ground. However, the air should be moving fast enough to be in fully turbulent flow to increase the heat transfer rate. To find when the air flow becomes turbulent you will need to calculate the Reynolds number, an online calculator can be found here. Another method to find an appropriate air flow rate is to base it off of the recommended air change rate of the greenhouse, or how many times the entire volume of air in the greenhouse should be circulated per hour. Recommendations on the best air changes per hour (ACH) vary widely but we have found consistent recommendations within 5-10 ACH. More information on flow rate and fan sizing can be found in the fans section.

 

CLIMATE BATTERY TUBING

The major considerations around climate battery tubing are material, length, depth, diameter and spacing. As a climate battery primarily consists of tubing with fans, each of these factors are important for efficiency. They will also be the primary drivers for cost.

 

MATERIALS CONSIDERATIONS

A common material for climate battery tubing is corrugated Advanced Drainage System (ADS) pipe. The tubing should also be perforated (designed with small slots or holes) to drain water built up from condensation. ADS is used because its characteristics make it an ideal material for geothermal uses. The pipes can be joined by heat, creating leak-proof fusion joints. They are also flexible and durable (very high stress resistant). It is important to follow the installation specifications provided by the manufacturer.

 

TOTAL LENGTH CONSIDERATIONS

The total length of the tubes that form the climate battery will be determined based on two main considerations. The first one is the optimal heat exchange between the earth and the air, which tends to increase with the length. The second one is the cost of installation and fan energy consumption, both of which also increase with an increase in tube length.

A common recommendation for length is between 25 and 35 feet. However, because the optimal length will change depending on the specifics of your climate battery, no clear recommendation can be made for general use. The tubing length should be matched with the selected fans to ensure that they can push air efficiently. More information on this can be found in the fans section.

 

DEPTH CONSIDERATIONS

Important considerations for the burial depths of the climate battery tubing include:

• The temperature variation of the soil.
• The water table.
• The increase in the resources needed to install the system.

 

THE TEMPERATURE VARIATION OF THE SOIL

Data on the temperature variation of the soil in the United States can be found from the Soil Climate Analysis Network (SCAN). This dataset is collected by the United States Department of Agriculture from multiple locations around the country. One Community’s open source thermal lag research is also a good source for better understanding soil temperatures.

If there is no data available, then you can use the approximation that 1.5-2 meters into the ground, the temperature remains constant throughout the year at the local annual average temperature. Other sources recommended shallower depths, anywhere from 2-4 feet, as long as the tubes are buried above the water table.

Research suggests that even deeper burial depths may be more effective. The figure below, obtained from the computational model developed by Da Silva et al, shows the variation of the outlet temperature of a climate battery (called an Earth-Air Heat Exchanger in the paper) throughout a year for 5 different burial depths. For our purposes, the greater the difference between inlet and outlet temperature, the better. The best results are observed for 5 meters (16.4 ft), however, we can observe that for burial depths between 3m and 5m (9.8ft and 16.4ft) the differences are only 1°C – 3°C. Therefore, a burial depth of 3m (9.8ft) would be adequate. See the section on soil for additional information regarding the influence of soil type, burial depth, and water content.

 

THE WATER TABLE

Your water table is another important consideration. As stated above, your climate battery must be buried above the water table. This is important because burying the system below the water table will mean that it is sitting in completely saturated soil. This soil will have drastically different thermal properties compared to unsaturated soil and will not store heat as effectively. To determine your water table you can access a public database or directly measure a shallow well. Database’s include: the USGS Groundwater Data for the Nation and the National Ground-Water Monitoring Network.

 

INCREASED RESOURCES TO INSTALL YOUR SYSTEM

How deep you install your system will affect the resources required for your climate battery. An increase in depth will also increase the cost of your system as you’ll need to pay for deeper excavation and more tubing. Excavation costs vary between companies, but you can expect to pay $50-$200 per cubic yard of dirt.

 

TUBING/PIPING DIAMETER CONSIDERATIONS

The diameter of the tubing is important because it will affect the flow velocity and heat transfer. Turbulent flow with a Reynolds number (Re) > 4000 is ideal for heat transfer. As seen below, the Reynolds number of a fluid depends on the viscosity of the fluid (a fluid’s resistance to flow), speed of flow, and the diameter of the pipe.

 

 

That being stated, you can generally use whatever diameter you want, as long as you get turbulent flow. However, 4in diameter tubing seems to be standard throughout the different climate batteries. The airspeed for 4in tubing should be greater than 125 fpm (0.635 m/s).

 

TUBING/PIPING SPACE CONSIDERATIONS

Little research exists around the optimal spacing between tubes. It is suggested to give 9-12 inches (.2286 m – .3048 m) between each tubing exchange layer.

 

SOIL

Soil types also play an important role in the climate battery system. Different types of soil have different compositions and hold different amounts of water and therefore, they have different thermal conductivity levels. It is important to keep the thermal conductivity of soil in mind when building underground and/or constructing climate batteries. Below is a chart for understanding the thermal conductivity differences of various soil types.

It is recommended that the soil be no more than 25% clay, as too much clay will build up an insulating layer around the pipes and limit heat transfer. If the soil has greater than 25% clay, mix the native soil with sand or a soil with low clay content and backfill the excavated areas with this new mix to improve heat transfer away from the pipes. A sandy loam soil is considered a great choice for climate batteries because of it’s thermal properties and nutrient content for plant growth (beneficial in cases of climate batteries being built below greenhouse structures).

 

SOIL TEMPERATURE

Soil temperature varies from month to month as a function of incident solar radiation, rainfall, seasonal swings in overlying air temperature, local vegetation cover, type of soil, and depth in the earth. Due to the much higher heat capacity of soil relative to air and the thermal insulation provided by vegetation and surface soil layers, seasonal changes in soil temperature deep in the ground are much less than, and lag significantly behind, seasonal changes in overlying air temperature. Thus in spring, the soil naturally warms more slowly and to a lesser extent than the air and by summer, it becomes cooler than the overlying air and acts as a natural sink for removing heat from a building. Likewise in autumn, the soil cools more slowly and to a lesser extent than the air and by winter, it is warmer than the overlying air and acts as a natural source for adding heat to a building. See One Community’s open source thermal lag research on Thermal Lag for a deeper understanding.

At soil depths greater than 30 feet below the surface, the soil temperature is relatively constant and roughly corresponds to the water temperature measured in groundwater wells 30 to 50 feet deep. This is referred to as the “mean earth temperature.” Figure 1 shows the mean earth temperature contours across the United States. In Virginia, the mean earth temperature ranges from 52º F in the northern Shenandoah Valley and Winchester area to 62º F in coastal Tidewater.

The amplitude of seasonal changes in soil temperature on either side of the mean earth temperature depends on the type of soil and the depth below the ground surface. In Virginia the amplitude of soil temperature change at the ground surface is typically in the range of 20-25º F, depending on the extent and type of vegetation cover. At depths greater than about 30 feet below the surface, however, the soil temperature remains relatively constant throughout the year, as shown below in Figure 2.

Deeper soils not only experience less extreme seasonal variations in temperature, but the changes that do occur lag farther behind those of shallower soils. This shifts the soil temperature profile later in the year, such that it more closely matches the demand for heating and cooling. Referring to Figure 3 for example, the maximum soil temperature occurs in late August (when cooling demand is high) at a depth of 5 feet below the ground surface, but occurs in late October (after the heating season has begun) at a depth of 12 feet below the ground surface.

Thus a deeper ground loop installation would lower the annual operating cost of electrical energy for running the heat pumps. Over the life of a ground heat pump (GHP) system, these accumulated savings may more than offset the higher capital cost of burying the ground loop more deeply. In order to determine the optimal depth of burial, it is important to know how the seasonal changes in soil temperatures vary with depth; this is mainly determined by the soil’s thermal properties.

 

SOIL THERMAL PROPERTIES

Heat capacity is the amount of heat required to raise the temperature of an object by 1 degree Celsius. For example, if a metal chair sits in the bright sun on a hot day, it becomes hot to the touch quite quickly. An equal mass of water under the same sun exposure will take longer to become nearly as hot. This means that water has a high heat capacity, i.e., the amount of heat required to raise the temperature of water is greater than a metal chair. The heat capacity of dry soil is about 0.20 BTU per pound per ºF of temperature change, which is only one-fifth the heat capacity of water. Therefore, moist or saturated soils have greater heat capacities, typically in the range of 0.23 to 0.25 BTU/lb/ºF. As shown in Figure 3 above, light dry soils experience greater seasonal temperature swings at a given depth than wet soils. This is because the water in the wet soil is able to maintain its temperature because it takes a lot of energy to change the temperature of water.

Thermal conductivity is another soil property that must be known in order to design a closed-loop or direct expansion GHP system. Direct expansion is a type of closed-loop system that doesn’t use a heat exchanger to transfer heat, but rather uses highly conductive pipe material, like copper, and directly transfers energy to the earth or soil. Thermal conductivity indicates the rate at which heat will be transferred between the ground loop and the surrounding soil for a given temperature gradient. The thermal conductivity of the soil is the critical value that determines the length of pipe required, which in turn affects the installation cost as well as the energy requirements for pumping working fluid through the ground loop.

Figure 4 indicates the thermal conductivity of different soil types. Heat transfer capability tends to increase as soil texture becomes increasingly fine, with loam mixtures having an intermediate value between sand and clay. As also shown in this figure, the thermal conductivity of any soil greatly improves if the soil is saturated with water. This effect is much greater for sandy soils than for clay or silt, since coarse soils are more porous and hold more water when wet. Therefore, groundwater level is another important site factor in evaluating a potential GHP project and optimizing the depth at which horizontal and spiral ground loops should be installed.

 

INSULATION

As previously stated, thermal mass is most appropriate in climates with a large diurnal (day-to-night) temperature range. For One Community’s specific location, high thermal mass construction with high insulation is desirable since the diurnal range/fluctuation is over 50º F (10° C). An ideal combination of thermal mass and insulation can be used to provide required thermal comfort very economically. When maximum heating efficiency is desired, it’s highly recommended to add EPS (Expanded Polystyrene) or a similar insulator under and around the sides of your greenhouse and climate battery system. This will effectively reduce heat loss to the surrounding soil and increase the climate battery’s heat capacity. Also consider the thermal mass of the greenhouse construction; a greenhouse with a concrete backing will store considerably more heat than a fully plastic construction. Adding thermal masses, such as black-painted barrels filled with water, can also be an effective thermal storage technique. However, making clear recommendations on thermal mass is difficult because solutions such as water barrels use space, money, and may not be needed depending on the construction of the greenhouse.

 

FAN SELECTION

A climate battery with the wrong fans installed will run inefficiently and will cause the fan to burn out more quickly, ultimately leading to worse heat transfer and higher costs. Choosing the right fans is therefore important and comes down to selecting a fan type, knowing your desired air flow, considering the static pressure of the system and, of course, matching the fan diameter to your tubing diameter (to ensure a proper fit upon installation).

 

FAN TYPE

There are two main types of fan to consider for a climate battery: axial fans and centrifugal fans (blowers). The main difference between the two is that axial fans are able to move a large volume of air, measured in cubic feet per minute (CFM), but maintain a low pressure. Centrifugal fans tend to have low CFM but produce a greater pressure differential.

For One Community’s application, the internal volume of the structure is very large and the air needs to be refreshed often, therefore requiring a high CFM. However, since the air needs to flow through a considerable length of pipes, there is a lot of pressure loss caused by friction. This means that for there to be air flow, the fan needs to produce a considerably large pressure differential. For climate batteries, it is recommended to use in-line axial fans.

 

TOTAL AIR FLOW

The ACH, or air changes per hour, is the number of times the air in a structure needs to be recirculated. The CFM determines how many cubic feet can be moved or exchanged each minute and this refers to the air in the structure.

ACH =CFM*60/Volume

Recommended ACH should be between 5-10. To Find the CFM, the equation above is rearranged into:

CFM =ACH*Volume/60

 

STATIC PRESSURE

Static Pressure is the resistance to airflow (friction) caused by the air moving through a pipe, duct, hose, filter, hood slots, air control dampers, or louvers. Static pressure is measured in inches of water gauge (SPWG), or in the metric equivalent it is rated in Pascal’s (Pa). This should include the pressure drop through all of the ductwork on the inlet and outlet of the fan or blower plus the pressure drop through any filters, control dampers, louvers, and other system components that restrict airflow.

When selecting a fan, it is important to understand its corresponding characteristic curve such as the one illustrated in Figure 1. By drawing a vertical line from the CFM at which your fan will be providing for your system, you will find the corresponding static pressure. Fans create “positive” static pressure while all other components in your system disturbing the air flow cause “negative” static pressure. The fan should be selected based on this curve and the pressure drop calculated from the tubing.

As previously stated, pressure drop is affected by the friction within the pipe and therefore, “friction loss” and “pressure drop” can be understood as the same thing. The Friction Loss Chart in Figure 2  will provide the friction loss per 100 ft of a tubing given that at least two of the following properties are known: Air Velocity (FTM), Air Volume Flow (CFM), or Duct Diameter (in).

To determine the friction loss per-unit length you must determine the initial velocity and size of the first section for this velocity. Table 6.1 can be used to determine a suggested initial velocity based on the CFM of the system.

The following equation will approximate the pressure drop per 100ft of tubing.

 

CONTROLS, SENSORS, AND AUTOMATION

Maintaining a controlled environment is important for plant health. A matter of hours in the wrong environment can cause plants to become significantly damaged or even dead. Having the proper sensors and controls can reduce labor, save energy, and increase plant health and yield. There are many existing available options from industrial systems to amateur DIY solutions.

The best system for your project will depend on your specific needs. We therefore recommend doing research on what will work best for you. Linked below are additional resources depending on what kind of setup you are looking for.

Here are the sensors and automation plans for our aquapini and walipini designs:

Coming…

 

HEATING AND VENTILATION

The climate battery will provide both heat and ventilation, however in most climates this will not be enough and extra heating and ventilation will be required. This guide draws directly from heating and cooling handbooks that serve as HVAC industry standards, specifically ASHRAE and ANSI.

 

DESIGNING FOR HEATING

The major sources of heat loss occur from conduction, radiation, convection, and infiltration (outside air entering the greenhouse). Using simple equations from ASHRAE, the total heat loss can be found by summing two equations.

Heat loss by radiation, conduction, and convection (q_c) and heat loss by infiltration (q_i) can be determined by the following:

q_c=UA(t_i-t_o)
q_i=.5VN(t_i-t_o)

Giving the total heat loss (q_T) as:

q_T=q_c+q_i

Where:

U = overall heat loss coefficient, W/(m²*K) (see Table 2,3)
A = exposed surface area, m²
t_i = inside temperature, C
t_o = outdoor temperature, C
V = greenhouse internal volume, m³
N = number of air exchanges per hour (see Table 4)

Maximum design heating load should be based upon the inside temperature required by the plants at night and an expected coldest night temperature.

Once you’ve found the total heat loss for your greenhouse, it is recommended that you size a heater to fully compensate for that loss. The climate battery will cover much of your heating requirements but is not reliable enough in cold and cloudy conditions. Of course, you can also omit a heater and plant only crops that will survive cold snaps.

 

DESIGNING FOR VENTILATION

The climate battery will provide a significant amount of cooling but the system does have its limitations. Specifically, during an extended period of hot days, the soil surrounding the system can reach near the same temperature as the inlet air, making any heat transfer ineffective. We therefore recommend building in adequate ventilation. An air exchange rate (the number of times the total air volume in a space is completely removed and replaced in a given time span) of 0.75 to 1 change per minute has been proven to effectively control the temperature rise in a greenhouse (Figure 7). We recommend achieving this through a combination of natural and mechanical ventilation and thinking of the climate battery as added efficiency.

Additional passive cooling included in the aquapini and walipini designs is underground piping that runs from the central pond into the structures. On hot days when venting causes negative pressure within the structures, this piping draws cool air from the pond surface and further cools that air through its passage underground.

Calculations show… [Research needed]

For One Community’s Walilipini #3, calculations show a heat loss of approximately -250,000 BTU/hr in peak winter conditions. This includes 230,000 BTU/hr from conductive losses and 20,000 BTU/hr from infiltration losses.

The recommended method for finding the cooling load is the CLTD/SCL/CLF method; a one-step, hand calculation procedure.

It may be used to approximate the cooling load corresponding to the first three modes of heat gain (conductive heat gain through surfaces such as windows, walls, and roofs; solar heat gain through fenestrations; and internal heat gain from lights, people, and equipment) and the cooling load from infiltration and ventilation. The acronyms are defined as follows:

CLTD”Cooling Load Temperature Difference
SCL”Solar Cooling Load
CLF”Cooling Load Factor

Go to page 28.39 of this handbook to find details of this calculation procedure.

SOLAR RADIATION HEAT GAIN CONSIDERATIONS

The following is a set of equations meant to give a better understanding of the solar radiation heat gain and effectiveness of the climate battery at cooling.

First we will need to find the maximum solar heat gain (Q) and then design the system around that. Q can be determined from the following equation:

Q=QSun*Aroof*cos(roof-summer)*ts

Where:

QSun = maximum solar radiation, can be found via a Report or Calculator, W/m²
Aroof = area of the greenhouse roof
roof = the angle of your roof from parallel with the ground
summer = the angle of the sun at the summer solstice = (90-Latitude+23.5), or use a Calculator
ts = transmissivity of your greenhouse glazing material

Once Q is found, the flow rate of air needed to cool the greenhouse can be determined by the following:

V = Q/(cp(ti-to)p)

Where:

V= the volumetric flow rate, m³/s
cp = 1 = the specific heat of air, kJ/kg⹦K
ti= the inlet air temperature, C (Maximum allowable greenhouse temperature)
to = the outlet air temperature, C (Temperature of the earth, ~55F)
p = the density of air, kg/m³
Q = solar gain at a maximum, kW

Calculations for our structures show… [Research needed]

 

VALIDATION

In order to quantify the effectiveness of One Community’s climate battery design, a team of engineers have worked on validating the design through various modeling approaches. The models will give a useful approximation of the system and its effectiveness.

• 2d Modeling of the Climate Battery
  • Summary of 2d Findings
• Evaluation of Boundary Conditions
• Simulation
• 3d Modeling of the Climate Battery
  • Simulation Parameters/assumptions
  • Cold Condition at West Jordan, UT
  • Hot Condition at West Jordan, UT

 

2D MODELING OF THE CLIMATE BATTERY

This is the first 2D model (more research/modeling is needed/coming) to study the airflow inside the climate battery. For the first model many assumptions are made for simplicity and initial evaluation. More accurate assumptions will be considered later.

Assumptions:

• Soil and inside air are separated and cannot exchange heat at the boundaries. The heat is just exchanged through the climate battery pipes. This assumption is to study the role of the pipes only.
• Top surface of the green house has been assumed to be 100°F.
• Top surface of the soil is assumed to be constant 80°F.
• Bottom surface of the soil domain is assumed to be 70°F.
• The thermal resistance between the pipe and the soil is considered to be zero. (First we want to see an ideal case of zero resistance and see the “transfer” of heat from air to the soil. Then we’ll consider the effect of pipe walls’ thermal resistance.)
• The bottom, left, and right walls of the greenhouse are insulated. (More accurate thermal conditions will be added later. Now we want to evaluate the flow of the air.)
• One pipe is considered for heat transfer from the air to soil.
• The greenhouse has a volume of 3178 ft³ (90 m³).
• These are the functions of the fan:
  • The fan changes all air inside the greenhouse 20 times every hour.
  • Volume change rate: Vdot = 1060 CFM (0.5 m³/s)
  • Velocity inside the pipes: 6.5 ft/s (2 m/s)
  • Conditions are steady.

This is the air velocity inside the greenhouse:

Air flow vectors inside the pipe:

Air flow vectors inside the greenhouse:

The air flows from the inlet at the top right side of the greenhouse to the outlet at the bottom left side. Air velocity inside the pipes are an order of magnitude bigger than the air velocity inside the greenhouse.

Temperature variations inside the climate battery:

Temperature of the air:

Temperature of the soil:

 

SUMMARY OF 2D FINDINGS

As can be seen, the way air enters the climate battery pipe transfers the heat to the soil. The cold air enters the greenhouse from the top right side. The top left side is the hottest location and the top right side is the coldest location of the greenhouse. The air enters the pipe with an average temperature of 35°C (96°F) and exits with an average temperature of 34°C (93°F). The average temperature of the greenhouse is 35°C (95°F). These results are limited for a few reasons. First, they are running assuming steady state conditions, in which the heat transfer rates remain constant, and therefore omit the dynamic state in which the climate system will actually run. Second, the boundary conditions are initial assumptions and are simplified. The next section will evaluate the boundary conditions in an attempt to refine these results.

 

EVALUATION OF BOUNDARY CONDITIONS

Before extending the simulation of the airflow, we need to evaluate the boundary conditions to obtain more accurate values that are closer to actual applications. For this purpose, we assume the following simple geometry for the greenhouse and the soil beneath it:

This is to capture more depth of the soil where we can assume that the temperature remains constant throughout the year with negligible error. Here we assume that at a burial depth of 50 ft the soil temperature is constant. We assume the following conditions:

• A natural convection is assumed at the outside of the greenhouse walls.
• The outside air temperature is obtained from the local seasonal temperature changes and climate conditions.
• The surface soil temperature is assumed to be the same as the outside air temperature.
• The floor of the greenhouse is assumed to be concrete.
• The thermal resistance due to the glass on the ceiling and walls at the left and right side of the greenhouse are neglected.

To obtain the values and show the design process, the conditions of the climate battery for the aquapinis is based on average variations of Los Angeles during January. The climate variations are obtained from Weather Spark (the conditions for any day/month/year and location can be obtained here). Based on the following figure, we can assume an average outside air temperature of 68°F during the days and 48°F during the nights in January.

 

SIMULATION

These simulations were created using weather parameters based on the Los Angeles area. If you’re considering running your own simulations, personalize the parameters to your location and greenhouse. Considering average conditions for 12 hours a day, in January, and simulating the transient thermal conditions, the temperature profile will be:

As can be seen, since the average outside temperature is close to the assumed temperature value in the deep soil, the temperature is almost uniform inside the soil with the soil at the bottom of the greenhouse being around 67°F. The temperature inside the greenhouse increases from an initial value of 68°F (assumed) to a value of 105°F after 12 hours of solar radiation on the ceiling during the day if the air stays still. Ventilation and the climate battery must be used to take this heat load off the greenhouse and keep its temperature constant. The boundary conditions from this simulation can be used in the previous 2D modeling for more accurate results.

Now, we want to see the effect of outside temperature and solar heat variations during a day in January. Since this is a transient simulation, the effects of transient boundary conditions are considered. For this purpose, the following problem is assumed:

In this model, only 30 ft of soil is considered. A concrete floor is considered at the bottom, the side walls are made of brick, and the ceiling is made of glass. First, a steady state thermal condition is assumed to achieve an initial temperature for the transient model (this helps to obtain more accurate results with fewer cycles). For the steady state, an average air temperature is considered, and a constant temperature is assigned at the 30 ft depth of the soil. The following is the result:

This shows that on an average sunny day in January the greenhouse will maintain its temperature to a 66°F (19°C) minimum which is above the 60°F (15.5°C) required by the plants.

 

3D MODELING OF THE CLIMATE BATTERY

A 3D model of a section of the climate battery system was created and put through winter and summer simulations (seen below). These CFD (computational fluid dynamics) simulations do not give an accurate representation of how the system will perform over an extended period of time, as it does not account for temperature variation within the day, heat storage within the soil, and other effects like wind and thermal contact resistance.

The simulations do show that the system will cool the greenhouse well in the summer. Inlet air at 100°F will be cooled to 60°F and pumped back into the greenhouse, providing a reliable source of cooling.

The winter simulations show that the climate battery will have limited effectiveness in the winter because of the cool soil temperature. Inlet air at 60°F will return at 45°F. It should be noted though that this does not take into account the heat storage that will occur on sunny days even in the winter. Additional research is needed to properly model the effectiveness of a climate battery in the winter. Modeling this system is difficult because of its dynamic nature.

One example of this is clear when considering the temperature of the soil surrounding the climate battery tubing, which directly impacts the rate of heat transfer between the soil and the air in the tubing. The soil temperature is dependent on the temperature of the surrounding soil and the temperature of the air running through the tubing. However, the air in the tubing is also dependent on the temperature of the soil and many other factors like the time of day. This kind of interdependent and time-dependent problem is difficult to solve with traditional thermodynamic models. [Research needed]

 

SIMULATION PARAMETERS/ASSUMPTIONS

Tubing Material: HDPE (high-density polyethylene)
Inlet Velocity = 250 CFM
Outlet Pressure = 81.2 KPa @6000 ft
Soil Properties = Dry Sand – Ran against “Dry Sand” and “Stiff gray brown sandy gravelly CLAY” with similar results

 

COLD CONDITION AT WEST JORDAN, UT

Temperature = 22°F
Humidity = 50%
Soil Temperature Gradient – Cold (Mid January)

 

HOT CONDITION AT WEST JORDAN, UT

Temperature = 92°F
Humidity = 95% at hottest
Soil Temperature Gradient – Hot (Mid August)

 

ONE COMMUNITY’S CLIMATE BATTERY DESIGN

 

TUBING

One Community’s climate battery will have two layers of tubing. Each layer will have 6 separate systems with one fan per system, for a total of 12 fans, as illustrated above. This number of fans gives an adequate flow rate for the greenhouse.

A flow rate of 250 CFM per system for a total of 3000 CFM has been selected. For a 33,000 ft³ greenhouse this CFM gives an air change hourly rate (ACH) of 5.4, which is within the rule of thumb range of 5-10.

The tubing will be HDPE with 12in diameter manifold tubing (a collection of parallel pipes that facilitates the transportation of air) and 6in diameter tubing for the rest. The tubing systems will be slightly different based on their location in respect to the greenhouse. The systems closer to the greenhouse structure will have 7 tubes and the farther systems that will have 6. This is to compensate for the extra length and consequent pressure drop.

This difference will equalize the total length of tubing and therefore equalizes the pressure drops between the two systems. Having a similar pressure drop means the two systems should perform the same, which is beneficial for maintaining and monitoring.

System 1 (near)				System 2 (far)
# of tubes	cfm per tube	tubing length		# of tubes	cfm per tube	tubing length
7	30.6	203.568		6	35.7	174.48

This configuration gives a maximum pressure drop of 5 inwg (inches water, a standard unit of pressure in the HVAC industry). More information for pressure drop and tubing length calculations can be found in the resources below.

 

BURIAL DEPTH

Below is an image of the SCAN database for Circleville, UT (a site at 6120 ft elevation) which provides information on the hourly air temperature and hourly soil temperature at -2in, -4in, -8in, -20in, and -40in (~1m). As previously mentioned, the necessary conditions for the appropriate depth to bury the climate battery tubes are:

• Difference in soil temperature at surface and underground temperature is greater than 10°F.
• Underground temperature needs to be constant year-round.

These conditions are achieved around -3.3 feet (-40 inches), so the climate battery tubes should be buried at approximately 3.3 feet below the surface.

 

CONTROLS, SENSORS, AND AUTOMATION

 

THERMOSTAT

• Circulation Fans – Connected to thermostat
  • One side a passive louver/shutter will open when the exhaust fan is in operation
• Climate Battery
  • Ground/air temperature sensor

 

HUMIDITY SENSORS

• Sprayer
  • Humidity sensor hooked up to an electric solenoid from a garden hose connected to a misting line
• Humidity sensor
  • Electrical
  • Waterproofed and grounded
  • Silicon wiring with temperature rating >150°F

 

ONE COMMUNITY’S CLIMATE BATTERY BUILDING PLAN

We have two options based on all the above research. Option one is to expect the climate battery system to work perfectly regardless of the lack of an accurate mathematical model and based on the assumption that it has worked great for many other people (see the examples of Working Climate Batteries section above).

If we go this route, we’ll build the first structure and climate battery and gather data to validate its overall effectiveness and cost effectiveness. Then we would decide if we’d like to install it in the remaining greenhouses.

The second option is to continue to run further analyses so we can validate the model sufficiently to feel more comfortable building the complete Aquapini/Walipinis plan. These additional analyses and simulations will be based on…

 

MAXIMALLY-AFFORDABLE WALIPINI
GREENHOUSE DESIGN ELEMENTS AND PLANS

Here are some extra non-climate battery information that should be understood and need to be further researched for development/integration into the final design.

 

GREENHOUSE DESIGN ELEMENTS

The figure below shows the minimum and maximum seasonal temperature changes of different areas in the US. Also, the following tables show the minimum and maximum temperatures recorded for different states in the US.

 

PLANT GROWTH

In terms of the growth of plants, the best soil for a greenhouse is a mix of loam soil with compost and potting mixture. To be specific, loam soil is the composition of sand, silt, clay, and humus. Humus functions to help the growth of vegetables and contains organic elements including decayed leaves and animal fertilizer to retain moisture and nutrients.

The optimal proportion of soil mix is 60% loam soil, 30% compost, and 10% potting mix. If the quality of local soil is not qualified because of its relatively low nutrients, a soil mix made of half potting mix and half compost mixture would be sufficient for greenhouse plant growing.

 

OTHER ENVIRONMENTAL CONTROLS

 

HUMIDITY CONTROL

At various times during the year, humidity may need to be controlled in the greenhouse. When the humidity is too high at night, it can be reduced by adding heat and simultaneously ventilating. When the humidity is too low during the day, it can be increased by turning on a fog or mist nozzle.

 

WINTER VENTILATION

During the winter, houses are normally enclosed tightly to conserve heat, but photosynthesis by the plants may lower the carbon dioxide level to such a point that it slows plant growth. Some ventilation helps maintain optimum internal carbon dioxide levels. A normal rate of airflow for winter ventilation is 10 to 15 L/s per square meter of floor area.

 

AIR CIRCULATION

Continuous air circulation within the greenhouse reduces still-air conditions that favor plant diseases. Recirculating fans, heaters that blow air horizontally, and fans attached to polyethylene tubes are used to circulate air. The amount of recirculation has not been well defined, except that some studies have shown high air velocities (greater than 1.0 m/s) can harm plants or reduce growth.

Vertical closed-loop earth heat exchangers are installed in boreholes 200 to 300 feet deep, where seasonal changes in soil temperature are completely damped out. Well-based open-loop systems also extend to this depth or deeper. These ground loop configurations are thus exposed to a constant year-round temperature.

On the other hand, horizontal-loop, spiral-loop, and horizontal direct-expansion (DX) loops are installed in trenches that usually are less than 10 feet deep. For these types of ground loops, it is important to accurately know the expected seasonal changes in the surrounding soil temperature. The extra cost of installing systems in deeper trenches may be outweighed by the gain in thermal performance since deeper soils have less pronounced seasonal temperature changes and are thus closer to room temperature, which reduces the work load of the heat pump units.

 

NATURAL VENTILATION

The principles of natural ventilation are explained in Chapter 16 of the 2013 ASHRAE Handbook”Fundamentals. Its suitability as a primary means of cooling must be judged and designed based on local environmental conditions, type of crop grown and the design of the greenhouse.

Radiation energy exchange is also important. Solar gain can be estimated using the procedures outlined in Chapter 18 of the 2013 ASHRAE Handbook”Fundamentals. Radiation loss must also be calculated and accounted for.

 

RESOURCES

• One Community Climate Battery Website Info – Resources
• Climate Battery Greenhouse: Energy Efficient and Sustainable Winter Growing
• Eco Systems Design Inc. Climate Battery Functions
• Eco Systems Design Inc. Climate Battery Calculator Version 2.0
• Low Grade Geothermal Heating and Cooling for Greenhouses – ZipGrow Inc.
• Greenhouse Forum at Permies.com: Small, above ground climate batteries?
• Resource of Resources: Their Consulting – Atmos Greenhouse Systems
• Ground to Air Heat Transfer – GHAT Greenhouse System – Ceres
• Garden Web Discussions: Is anyone using a shcs in a cold climate?
• Garden Web Discussions: Any updates on data from those with SHCS greenhouses?
• Garden Web Discussions: Subterranean Heating/Cooling System
• Renewable Energy Solutions – Jumping Cable Solar Homes
• KnowledgePublications – Heating and Cooling Your Home
• Citrus in the Snow – Geo Air Technology
• Building a Solar Greenhouse with the Subterranean Heating and Cooling System
• Passive Solar for Heating and Air Treatment
• Article: “Working Parameters Affecting Earth-air Heat Exchanger (EAHE) System Performance for Passive Cooling”
• Definition: “Reynolds Number”
• Greenhouse Ventilation – UF askifas
• Threefold Farm – Climate Battery Greenhouse Version 2
• Eco Systems Design Inc. Climate Battery FAQ
• The Leader in High Altitude Forest Gardening and Near Net-zero Greenhouse Design for Over 30 Years
• Threefold Farm – Climate Battery Greenhouse Version 1 aka The Gray House
• Chapter 13: HVAC Applications for PE Pipe
• ADS Single Wall Heavy Duty Pipe Specification
• NWCC Report Generator – Circleville (2125) Utah SCAN Site – 6120 ft Reporting Frequency- Hourly; Date Range- 2019-01-01 00-00 to 2019-01-31 23-00
• Article: “Study on Calculation Models of Earth-Air Heat Exchanger Systems”
• 10 Do’s And Don’ts For Designing A Ground To Air Heat Transfer System
• Article: “Improving the performances of earth air heat exchangers through Constructal design”
• USGS FAQ – How can I find the depth to the water table in a specific location?
• Home Advisor – How Much Does It Cost to Excavate?
• Wikipedia: “Reynolds Number”
• Food Autonomy w/ Evan Doukas/ How To Build A Climate Battery Greenhouse ” SOLE
• Climate Battery – An Improved Greenhouse Climate Solution
• Ground Temperatures as a Function of Location, Season, and Depth – Build It Solar
• Article: “What Is The Best Soil For Greenhouse Plants?”
• McGill AirFlow Duct System Design Guide First Edition
• Article: “Thermal Conductivity, Thermal Diffusivity, and Thermal Capacity of Some Nigerian Soils”
• How to Size Greenhouse Fans & Shutters – RIMOL Greenhouse Systems
• How to Read a Fan Performance Curve – G Squared Engineered Products
• Article: “Fans: Airflow versus Static Pressure”
• Air Density Correction Factors Table
• Fan Engineering – Temperature & Altitude Effects on Fans
• Designing Air Flow Systems – Air Flow, Air Systems, Pressure, and Fan Performance
• Fan Basics – Air Flow, Static Pressure, and Impedance
• SmartBee➢ Controllers – The Best Automated Grow Systems for Home
• Environmental Controls – Greenhouse Megastore
• Smart Greenhouse Climate and Control Systems – Postscapes
• 2015 ASHRAE Handbook: Heating, Ventilating, and Air-Conditioning APPLICATIONS
• 2018 ANSI/ASHRAE/ACCA Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems
• One Community Climate Battery Website Info – Resources – Heating and Ventilation 
• Chapter 28: Nonresidential Cooling and Heating Load Calculations
• USDA NWCC Report Generator 2.0
• Daily Solar Energy Calculator – Fabhabs
• Window Heat Gain Calculator
• Average Weather in January in Los Angeles, California, United States – Weather Sparks
• The Weather Year Round Anywhere on Earth – Weather Sparks
• Wikipedia: “High-density polyethylene”
• Article: “Determination of Thermal Conductivity of Coarse and Fine Sand Soils”
• Average Weather on January 15 in West Jordan, Utah, Unites States – Weather Sparks
• NWCC Report Generator – Circleville (2125) Utah SCAN Site – 6120 ft Reporting Frequency- Hourly; Date Range- 2019-08-15 00-00 to 2019-08-31 23-00
• USDA National Water and Climate Center: Soil Temperature (8 in.) Percent of POR Median November 29, 2022, end of day
• Records – State Climate Extremes Committee (SCEC) | National Centers for Environmental Information
• USA State Temperatures Mapped For Each Season – Current Results
• ASHRAE Handbook Volumes Online
• Use this page (click here) if you have a resource you’d like to suggest be added here.

CLIMATE BATTERY BOOKS

• The Year-Round Solar Greenhouse: How to Design and Build a Net-Zero Energy Greenhouse by Lindsey Schiller and Marc Plinke
• The Forest Garden Greenhouse: How to Design and Manage an Indoor Permaculture Oasis by Jerome Osentowski

 

SUMMARY

A climate battery is an innovative and sustainable design for heating and cooling. Using their geothermal energy system, climate batteries regulate air temperature and humidity and ultimately serve to provide more reliable, efficient, and sustainable heating and cooling solutions for greenhouses, homes, and non-residential buildings. We have researched many resources to make our designs replicable, durable, and easy to build. These open source tutorials will be used and evolved as we construct everything on this page and more as part of our Aquapini/Walipinis and Tropical Atrium. 

 

FREQUENTLY ANSWERED QUESTIONS

Q: Is this guide complete?

No, we do not consider this guide complete yet. We need to complete more research and even then this design will not be considered complete by us until we build our own climate battery and test it. We’ll share the plans and results when we have them. We will also share anything new we learn during the process.

• Use this page (click here) if you have a question you’d like to suggest be added here