Impetus

Over the 2018-2019 school year, PSU mechanical engineering students studied our two existing climate batteries to get an idea of how they functioned. Their results helped to begin to quantify the amount of heat stored and released into the greenhouse in cooling and heating cycles.

During the study, one of the most interesting pieces of information was that the airflow in the heat transfer tubing was unbalanced. This ultimately led to very unequal heat transfer across tubing and thereby decreased the overall heat transfer efficiency since the soil, acting as our “climate battery”, wasn’t interfaced in an equal way. Our impetus for 2019-2020 was to come up with a design that more properly interfaced with the soil.

Study Goals

Primary Goal: Determine the best design within the constraints of effective interfacing with the soil, balanced airflow, cost, ease of installation, and scalability

Secondary Goal: Determine performance characteristics of the system

The Process

At the beginning of the school year, the PSU team moved sensors in the blue house to study heat transfer within the blue house heat transfer piping. The blue house was much easier to install than the gray house, whereas the gray house seemed to perform better. Therefore, we wished to recreate the performance of the gray house with some of the design characteristics of the blue house.

Based on the data, the students came to the conclusion that most of the heat transfer in the blue house seemed to occur in the first half of the tubing. A decision was made to shorten the tubing in the new design both due to that data and the desire to produce a design that could work in a variety of sizes of greenhouse.

The team then worked with us at the farm to go through several iterations of design before finally deciding on a design with which to run airflow and heat transfer simulations. Designs were created and airflow simulations run through SolidWorks. The resulting information was then fed into Comsol for heat transfer simulations.

The Results

Air Velocity and Balancing

Faster air velocity and a more balanced airflow was achieved through the use of both an intake and exhaust fan. The study group from the prior year happened upon this and determined that airflow was better balanced through the use of baffles and an additional exhaust fan. It was proposed that both the intake and exhaust fans would be matched in flow rate and higher CFM fans would be preferred like those in the gray house. For the simulations, both intake and exhaust fans with a flow rate of 5,000 CFM were used. With this, the range of airflow for the final system was 38ft/s to 9ft/s through the tubing. This is still far from fully balanced, but the increased air velocity allows for more effective heat transfer, as is seen in the Comsol simulation data below. For comparison, data from the previous year’s study showed the airspeed in the gray house piping at a range of ~4 - 18 CFM.

Comsol Heat Transfer Simulation

A heat transfer simulation was done using a single length of heat transfer piping (4” corrugated perforated pipe) at a 22’ length, using a surrounding soil temperature of 50F. The simulated was run using 80F intake air. This would simulate well our conditions in February when we’re experiencing some solar gain and the soil is still relatively cool.

Air Velocity @ 24ft/s Results

Exit Temperature 67.5F
Heat Transfer Per Tube 1,680 BTUs/Hr
Reynold’s Number 49,024

Air Velocity @ 34ft/s Results

Exit Temperature 65.6F
Heat Transfer Per Tube 2,758.4 BTUs/Hr
Reynold’s Number 69,431

Air Velocity @ 48ft/s Results

Exit Temperature 59F
Heat Transfer Per Tube 5,650 BTUs/Hr
Reynold’s Number 98,048

Heat Transfer Simulation Thoughts

Increased airspeed seems to lead to a linear increase in heat transfer, likely up to a point. It’s unknown at this point which level of heat transfer is ideal. I would assume at some point that the heat transfer would max out at a certain velocity then perhaps begin to decline. Another unknown is the rate at which heat would diffuse into the soil surrounding the tubing and become “saturated” for the day. For now, 15” spacing on our heat transfer tubing gives us the greatest air velocity and therefore the greatest potential for heat transfer. In the future, more may be done to determine if even less tubing could be utilized with even greater heat transfer.

Improved System Design Specification

Assembled System Size - 24’ wide x 24’ long x 8’ deep

A system in the climate battery is defined as the combination of interconnected risers, manifolds, and heat transfer piping. Multiple of these systems can be installed per greenhouse depending on the length of the house. Each system of tubing: riser, manifold, and heat transfer tubing is 24’ long by 24’ wide and approximately 8’ deep (into the soil depth). This should allow for straightforward installation in a 30’ or 34’ wide greenhouse and possibly in a smaller one like 26’ if care is taken around the side walls. A fairly standard 30’x96’ house should easily accommodate three such systems and possibly four if some shared exhaust manifolds are utilized or some heat transfer piping runs are shortened slightly.

System Tubing Specifications

Riser: 24” diameter dual-wall corrugated piping as a 20’ section, cut in half for 2, 10’ sections to serve as the intake and exhaust risers

Manifolds: 4 sections of 18” diameter, 20’ long dual-wall corrugated piping, joined perpendicular to the risers at 3’ and 7’ below grade.

Heat Transfer Tubing: 4 individual layers of 15-16 tubes, each approximately 22.5’ long, attached at 15” centers on the manifolds. Separate layers to be installed at 2’, 4’, 6’, and 8’ below grade. The upper manifold carries two layers of tubing and the lower manifold the same. This equates to 60-64 tubes per system.

Fans: One intake and one exhaust fan, approximately 5,000 CFM (1/3 or 1/2 HP fans), both mounted vertically in the riser. The intake fan in a “push” orientation and the exhaust fan in a “pull” orientation.