Indoor farming is gaining popularity in India. The driving force behind its popularity is the latest farming techniques such as Hydroponics, Aquaponics and Aeroponics. All these techniques use soilless cultivation methods and provide nutrition through the water being circulated. Indoor farming offers several benefits such as:
- Year-round crop cultivation under controlled environment. Many crops which are specific to a particular area, can now be grown anywhere in India.
- Unaffected by outside weather conditions. Hence very less losses due to extreme weather conditions.
- Having farms near urban centers reduces transportation costs and provides fresh Agri produce.
- Higher productivity due to vertical farming. Requires very less water and no soil and does not use pesticides.
- Due to automation, less labour cost and does not need heavy machinery.
In India, cooling load is higher in summer season. For achieving economy of scale, these indoor farms are fairly big. Which need big capacity Refrigerators / Chillers. Hence, the initial investment is large and monthly electricity bills are high. There are cases where farmers have invested and incurred losses due to heavy running expenses. Therefore, there is a need to provide optimal cooling solutions. Here, a novel heat exchanger is proposed which will provide energy recovery from the CO2 laden cool exhaust air from mushroom grow chamber.
Indoor Mushroom Cultivation
White button mushrooms are being cultivated in India on a large scale. Initially it started as a cottage industry. Mushrooms are grown in winter season. Now, many farmers have started growing them round the year in hi-tech indoor farms under controlled environmental conditions. Temperature, Humidity and CO2 levels are controlled accurately to get good quality mushrooms.
Mushroom Growing Chambers: These are big in size measuring about 60 ft in length and 20 ft wide. Most chambers have height of about 20 ft to accommodate approximately 8 racks vertically. These racks extend throughout the length of the chamber. Chamber has two levels. Bottom four racks are accessible from the ground level and for top 4 racks, platform is provided at the middle level. Two passages are provided for people movement between side racks and the middle rack.
The walls and roof of growing chambers are thermally insulated using thick PUF sheets. Chiller unit of 11 Ton capacity is used for achieving required temperature levels in various growing steps. Blower in the air handling unit ensures uniform temperature distribution throughout the chamber. Louvers are provided for expelling inside air when CO2 level exceeds maximum limit. These louvers are usually opened twice a day, morning and evening for about 2 hours. At these times of the day the outside air is cooler.
One harvest of mushroom cultivation takes about 2 months. Thus, in one year 6 harvests are possible. For continuous mushroom production throughout the year, at least three such chambers are needed. For one harvest, following three steps are run inside the growing chamber in a span of about 2 months:
- Spawn running
- Casing
- Fruiting
Spawn Running: About 10 to 12 kg of compost with spawns (seeds) sprinkled, is filled in polyethene bags. These bags are arranged on the racks. Above growing chamber can accommodate about 2000 such bags. Therefore, there is a load of about 20 tons inside the chamber. Temperature of the bags is maintained at 24±2 °C during this phase. Which means the air temperature in the chamber should be set at one or two degrees lower. Relative humidity of 90% along with high CO2 concentration is maintained. In about 12 to 14 days, the mycelium (fungal threads) grows out from the spawns.
Casing: On the compost surface, a layer of cocopeat is deposited. This layer is watered regularly to maintain high levels of humidity. The bag temperature is again maintained at 24±20C and relative humidity of 85 to 90% is maintained for about 8 to 10 days.
Fruiting: Fruiting is induced by slowly lowering the temperature to 17±10C. Humidity is maintained at 85 to 90%. During fruiting, large amount of CO2 is generated along with some heat. If CO2 level exceeds 0.15%, the air needs to be flushed out and replaced with fresh air. The fruit body initials which appear as pin heads, later develop into button stage. Fruiting phase lasts for about 30 to 40 days.
As mentioned above, air needs to be flushed out at least two times in a day. For this purpose, exhaust vents are fitted with wire net, louvers and insulated lids. Inside the chamber, the CO2 being heavy, it settles down at lower levels. Hence, the exhaust vents are provided at few inches above the ground level. The louvers, when opened allows CO2 filled air to exhaust under positive pressure created by the blower from air handling unit. Every time the cool air is flushed out, there is loss of energy. The fresh air again needs to be cooled. During summer season, outside air can be above 40 °C, which puts heavy load on the chiller unit.
Best time to flush out the CO2 filled air is in the early morning and late in the evening. At these times, the outside air is not very hot and the fresh air will need less cooling energy.
Novel Heat Exchanger Design
Proposed air to air heat exchanger for energy recovery: As mentioned above, when the CO2 level in the chamber exceeds maximum limit, that air needs to be expelled out and replaced with fresh outside air. Usually, this process takes about one to two hours. Therefore, lot of energy is lost in the form of cold air. The outside air again needs to be cooled down, which consumes substantial amount of energy. It is possible to recover part of this energy by cooling the incoming fresh air. Here, a novel air to air heat exchanger design is given.
This heat exchanger uses a square aluminium pipe having fins inside. Figure 1 shows the cross-sectional view of the pipe. Seven fins inside the pipe form 8 sections. Through these sections air can be circulated throughout the length of the pipe without getting mixed. If warm air and cold air is circulated in alternate sections, then heat exchange takes place. In this way, warm air gets cooled.
Such a pipe can be manufactured using extrusion process just like heat sinks are manufactured for use in electronics. Usually, such extrusions come in standard length of 12 feet. Required length of pipe can be cut from this extrusion.
Figure 2 shows the top view of aluminium extruded pipe which has been cut to the required length. The cross-sectional view shows 8 sections numbered from 1 to 8. In even numbered sections, slots are made as seen in the top view. Sections 2, 4, 6, 8 shown in red colour are used for circulation of outside warm air. One end of pipe has four slots and other end has another four slots. A square plastic pipe (red) covers 4 slots for inlet of air. Four slots on the other end of the aluminium pipe are also covered with another square plastic pipe. These two short plastic pipes convert square cross section into round cross section at the other end (see Fig. 6). To the round cross section, longer extension pipes are attached as per the requirement.
Figure 3 shows the top perspective view of the pipe. It shows the two sets of slots. Slots shown with full red arrows act as inlet for outside warm air. The second set of slots shown with red/blue arrows acts as outlet after the heat exchange has
taken place.
Similarly, in Fig 4, bottom view of the aluminium pipe is shown. Here, slots have been made in the odd numbered sections. Sections 1, 3, 5, 7 circulate cold exhaust air. One group of four slots are for inlet of cold exhaust air and the other group is for outlet of exhaust air after heat exchange has taken place. These slots are covered with two square plastic pipes (blue). These pipes also have round cross section (square to round converter) at the other end, for fitting longer pipes as required.
Figure 5 shows the bottom perspective view of the pipe. It shows the two sets of slots. The slots shown with full blue arrows acts as inlet for cold exhaust air coming from chamber. The second set of slots shown with blue/red arrows acts as outlet after the heat exchange has taken place.
Figure 6 shows the heat exchanger assembly. To the aluminium pipe, four plastic pipes are attached. As mentioned earlier, these plastic pipes convert square cross section area to round cross section for fixing extension pipes.
At the bottom, two blue plastic pipes are fixed for circulation of cold exhaust air in odd numbered sections of the heat exchanger. Exhaust air after circulation inside the heat exchanger, is released to the atmosphere. On the top surface two red plastic pipes are fixed for circulation of fresh warm air. Once this air is circulated through the even numbered sections, it is sent to the mushroom chamber. This air being partially cooled, will reduce load on the chiller, thus saving significant amount of energy.
At the two ends of aluminium pipe, end plates are fixed to prevent air leakage. Please note that other accessories like extra pipes, fans for circulation of air, enclosure for the heat exchanger, air filters etc, are not shown in the figure. These accessories will have to be added as per the specific design.
Prototyping
The aluminium extruded pipe shown in Fig. 1, is not available in the market. To make a prototype for testing the performance of the heat exchanger, we can use heat sinks which are readily available in electronics shops.
Figure 7 shows the cross section of a heat sink, which is readily available in the market. Fig. 8 shows two such heat sinks attached to make a square pipe with fins inside. This can be used for making quick prototype for performance evaluation.
Conclusions
Proposed air to air heat exchanger is fairly simple to fabricate. The fins create several sections inside an extruded aluminium pipe for flow of cool and warm air side by side. Such a heat exchanger is very effective because, throughout the length of pipe the heat exchange takes place. If this heat exchanger is used for recovery of energy in a mushroom farm, it can save good amount of electric power consumed by the chiller unit. Which will improve the profit margin for the farmers, especially in summer season. The proposed design is simple and cost effective. It is a one-time investment, which will pay back itself within one or two harvesting cycles.
Dr. Vijay Deshpande has done PhD from IIT Kanpur in Electrical Engineering. He has worked in several companies in India. He worked as Technology Specialist and retired from Honeywell India. His current interests include working on cost effective Cooling Techniques and also on Solar Photovoltaic Systems. He has published several research papers in International Journals and published many articles in leading magazines.
