Reducing New England High Tunnel Heating Costs and Fossil Fuel Usage Utilizing an Earth Air Heat Exchanger

2020 Progress:

Due to complications with the COVID-19 pandemic, the project has had delays in certain areas, though there are still very interesting findings to report.  Heat has not been implemented in either tunnel yet as the boiler installation is still in work.  We hope to have the boiler fully functional soon, but it may not be until next winter.  Due to this delay, the only factors being monitored are air temperature and Relative Humidity (RH) in the Climate Battery Tunnel (T3), the non Climate Battery Tunnel (T4), and the Climate Battery Intake, and Exhaust ducts.  Measurements are also being taken for Soil Temperatures in T3 and T4.

Data was synthesized from November 17th to December 15th of 2020.  All four sensors that represent air temperature and RH in each tunnel were averaged together to create one data point.  The same was done for soil temperature, and Climate Battery intake and exhaust temperature and RH.  Data points were collected every minute for all measurements except for T3 Soil Temperature, which was collected every hour.  This was overkill and created an overly cumbersome project for synthesizing data.  Data points will be collected far less frequently moving forward to account for multiple years for of the project's life that will need to be synthesized.

Over the time surveyed, the Climate Battery tunnel showed an average daily temperature that was consistently warmer than the control tunnel by at least 0.5^o F, but as high as 6.6^o F.  The average temperature difference over the entire time surveyed was 3.6^o F warmer in the Climate Battery Tunnel (Tables 1 and 2).  When the entire data set was compared, the largest differences observed at any given time was -1.7 degrees colder in the Climate battery tunnel than the control, but as high as 12.4 degrees warmer.

_{Table 1. Daily Air Temperatures in the Climate Battery Tunnel (T3) and the Non-Climate Battery Tunnel (T4).}

_{Table 2. Degrees F warmer the average daily air temperature was in the Climate Battery Tunnel (T3) when compared to the Non-Climate Battery Tunnel (T4).}

Something of particular interest to Winter growers may come when analyzing daily low temperatures in each tunnel.  Between the dates of November 17th and December 15th, the Climate Battery Tunnel only had a low point temperature below freezing on 7 of the 29 days with the lowest temperature being 29^o F, while the non-climate battery tunnel had a low point temperature below freezing 19 of the 25 days with the lowest temperature being 22.3^o F (Table 3).

_{Table 3. Daily Low Air Temperatures between the Climate Battery Tunnel (T3) and the Non-Climate Battery Tunnel (T4).}

Soil temperature showed similar, though less drastic results.  The soil temperature in the Climate Battery tunnel was higher than the non-Climate Battery tunnel for 27 or the 29 days.  The highest average daily temperature difference was 4.2 degrees warmer in the Climate Battery Tunnel, the lowest average daily temperature difference was -1.6 degrees, and the overall average daily temperature difference was 1.8 degrees warmer in the Climate Battery Tunnel (Tables 4 and 5).

_{Table 4. Daily Average Soil Temperatures in the Climate Battery Tunnel (T3) and the Non-Climate Battery Tunnel (T4).}

_{Table 5. Degrees F warmer the average daily soil temperature was in the Climate Battery Tunnel (T3) when compared to the Non-Climate Battery Tunnel (T4).}

Perhaps the most surprising outcome to come from this preliminary data was that the average daily RH in the Climate Battery Tunnel was consistently lower than the non-Climate Battery tunnel.  Only one day of the 29 measured measured the Non-Climate Battery Tunnel lower in RH, by 0.1%, though the Climate Battery Tunnel had an average daily RH of 1.2% lower overall, with the highest difference being 3.5% lower (Tables 6 and 7).  This is especially surprising because when the Climate Battery starts introducing warm air into the cold tunnel, moisture condenses consistently, creating a fog-like atmosphere, especially in the early mornings.  This does not seem to create a large raise in RH in the Climate Battery Tunnel because this typically occurs when RH is around 99-100% in the control tunnel as well.  It has yet to be determined if this small drop in RH would have any impact on disease pressures without adding additional controls such as venting, though it appears that the Climate Battery will not pose an additional disease pressure in the form of increased RH.

_{Table 6. Daily Average Air RH% in the Climate Battery Tunnel (T3) and the Non-Climate Battery Tunnel (T4).}

_{Table 7. RH% higher the average daily air RH was in the Non-Climate Battery Tunnel (T4) when compared to the Climate Battery Tunnel (T3).}

Moving forward, the setpoint for the Climate Battery will be adjusted for the remainder of the Winter.  Up until now, fans would turn on to store heat in the Climate Battery when a thermostat hung from the center of the tunnel reached 80^o F or warmer, and shut off when lower than 80^o F.  Additionally, the fans would turn on to draw heat from the Climate Battery if the temperature dropped below 40^o F, and turn off when temperatures raised above that threshold.  Moving forward, the setpoint for the Climate Battery to store heat will be adjusted for the remainder of the Winter from 80^o F down to 65^o F.  This is due to air temperatures in the tunnel rarely getting above 80^o F, and average daily exhaust temperatures from the Climate Battery steadily dropping from 51.4^o F on November 17th, to 41.6^o F on December 15th (Table 8).  It is assumed that by banking heat of 65^o F or warmer, it will increase the Climate Battery's warming potential during Winter months, without too much of a deleterious effect on the overall max temperature during the day.  The reason for this is that the tunnel does not have to warm back up from such cold temperatures, and can reach its maximum temperature much higher than the non-Climate Battery tunnel.  The low-temperature threshold will not be adjusted at this time.

_{Table 8. Daily Average Temperature in the Climate Battery Exhaust, Tunnel 3, and Tunnel 4.}

Planned Materials and Methods

Treatments:

Two 30’X96’ Rimol Nor’Easter high tunnels will be used as treatments with one 30’X52’ Rimol Nor’Easter high tunnel utilized as a comparison. The treatments will be known by their on farm designation until the final report to avoid confusion. One of the 30’X96’ tunnels, Tunnel 4 (T4), will utilize only a radiant heating system, with hot water PEX piping installed in a serpentine pattern. Pipes are placed roughly 8” apart spanning the width and length of the tunnel at a 2 foot depth. Three separate zones will supply the tunnel with hot water fired from a spent cooking oil furnace in a separate building.

Tunnel 3 (T3) is the other 30’X96’ tunnel and will utilize an Earth Air Heat Exchanger (EAHE) in addition to the radiant heating system. The radiant heating system will be powered by the same furnace as T4. The EAHE was constructed by lining the perimeter of the high tunnel with 4’X8’X1” rigid insulation, starting from the top of the soil line to a four foot depth. The entire tunnel was excavated to about 4 and a half foot depth, and a base of crushed stone was laid roughly 3-6” thick. Running along the inside of each end wall, a 15” diameter rigid corrugated pipe was used as a manifold to connect all flexible lines. The manifold pipe was laid about 3’ from the end wall, running the width of the high tunnel. A 15” diameter pipe attaches to the manifold running vertically on each end. On one end is the intake, and on the other end is the exhaust. Attached to the manifolds on either end are 4” perforated, flexible pipes covered in a fabric “sock”, designed to send air throughout the crushed stone. There are 22 rows of 4” pipes, separated by about 16” each. The bottom layer is then covered with about a foot of crushed stone, and a layer of landscape fabric is laid over top. A second layer of the EAHE is then laid, identical to the first, but at about a 3.5’ depth. On top of the second layer of landscape fabric is where the radiant heating system is installed, followed by topsoil.

On either end of the tunnel, intake manifolds will run up to the top ridge of the tunnel to pull warm air from the highest point in the tunnel. Mounto 16” fans (SN – 190706060) will be placed in-line on each intake manifold to move air throughout the EAHE. Fans will be controlled by air temperature sensors at the peak, and 1 foot above the soil line. When peak temperatures exceed 75 degrees F, fans will turn on. When soil temperatures at the growing zone drop below 40 degrees F, the fans will turn on, unless EAHE temperatures drop below soil line temperatures, and will turn off when soil line temperatures reach 50 degrees.

Tunnel 2 (T2) is a 30’X52’ tunnel, identical in its design except for the length, and will have neither a radiant heating system, nor an EAHE. While not identical in size, the tunnel has a similar orientation, and can be used as a rough comparison. Finally, an outdoor (O) treatment will be prepared to measure ambient outdoor air temperature and relative humidity, as well as soil temperature.

Data collection:

Each treatment will collect data for soil temperature, air temperature and relative humidity. All high tunnels will utilize Agribon AG-19 fabric, covering beds about 6” off the ground in the winter to create an additional microclimate, exhaust fans to reduce maximum temperatures in the shoulder season, and roll up sides in the summer. Roll up sides will only be put up or down at the same time, and temperatures for the exhaust fans will all be set to identical temperatures. When fabrics are installed or removed, all will be installed or removed at the same time to reduce variability. Two pieces of fabric will be installed per tunnel, one covering 4 beds on the left side of the tunnel, and one covering 4 beds on the right side of each tunnel. Because of this, 2 sensors for air and soil temperature, and air RH will be placed on the right and left sides of the tunnel, one on each end, totaling 4 of each sensor per tunnel. During the winter, T3 will send the exhaust from the EAHE underneath the Agribon covering by way of a flexible heating duct. T3 will also measure air temperature and RH at the intake and exhaust of each EAHE duct. All data will be sent to a HOBO RX3000 Remote Monitoring Station Data Logger for automatic collection, and data will be sent to a cloud storage system to be analyzed at the end of the trial.

For T3 and T4, each of the 3 hot water PEX zones per tunnel will be equipped with an inline water meter to measure the amount of water being sent through the system. Each zone will also have temperature sensors taped to the outside of the incoming and outgoing PEX pipe. At the end of the trial, the average outgoing water temperature will be compared to the average incoming water temperature to determine the BTU requirements, and ultimately cost, of heating the amount of water used per tunnel. Soil and air temperature will be collected from T2 and outdoors for comparison.

Data will be sent from the Data Logger to the cloud storage system from June 2020 through June 2022. At the end of the trial, all data will be interpreted to average temperatures and relative humidity levels for different hours of each month. Average cost of heating for T4 and T3 will be compared to determine the efficacy of an EAHE in reducing tunnel heating costs.

Technical Advisors:

UNH Extension Field Specialist Jeremy Delisle and UNH Extension Field and Greenhouse Specialist Jonathan Ebba will act as technical advisors for this project. UNH Extension Sustainable Horticulture State Specialist Becky Sideman has assisted in experimental design for this trial.