Seaweed Farm Infrastructure Requirements

Seaweed cultivation is growing worldwide and is an exciting area of sustainable business. However, the cost and complexity of processing infrastructure is often an overlooked but important factor in the viability of business opportunities. Drying, in particular, is critical for any seaweed that will not be sold fresh locally. Drying drastically reduces shipping costs (both monetary and environmental), preserves the seaweed, and makes secondary processing easier.

Existing processing infrastructure for seaweed is typically negligible, relying on selling fresh or solar dehydration (sticking it in the sun until dry). This article will explore drying solutions that are environmentally friendly, weather-independent, and consistent. For each option, the capital and environmental costs will be examined. Ultimately, the “correct” decision depends on the goals of the project and the owner, but several of these options rise above the rest.

Model System – Fuqua Farms Example

In order to compare different drying methods, it was necessary to imagine an example seaweed farm that could be used to calculate energy and time costs. We will call it Fuqua Farms.

Fuqua Farms produces 1000 kg. of seaweed (wet) per harvest. They harvest seaweed there once per week, 52 weeks per year. They have a shipping container on the property that would be used for mechanical drying methods. Solar panels could be installed on the roof of the container to offset the energy of a mechanical drying system. 6 solar panels fit on top of this shipping container. These could produce around 3000 KWh per year. Fuqua Farms wants to make sure that they are at least net energy neutral for drying so any drying method they choose would use less energy than that per year.

Typical drying times for seaweed were estimated using secondary experimental data. 40% moisture content seemed standard and therefore drying times were calculated to that parameter. The estimated drying time and line of best fit are shown in Figure 1, below. It is highly recommended that this data be validated experimentally and calculations adjusted as needed once the seaweed farm is operational.

Figure 1: Seaweed Drying Time vs. Temperature

Methods

Natural drying methods are those that required no mechanical intervention, such as solar dehydration or solar chimneys. Mechanical drying methods use energy to move air, change air temperature, or remove moisture in order to speed drying. Mechanical methods that were considered included Unit Heaters, Air Handling Units, or Dehumidifiers.

Figure 2: Drying Methods Explored and Diagrams

Solar Dehydration:

Solar dehydration, or simply air-drying, is the easiest method. Already in use in all over the world, typically this involves hanging freshly harvested seaweed over racks on a sunny day. This method is zero input, much like a seaweed farm, and is therefore highly appealing from an environmental and economic standpoint. It is best suited for the individual farmer selling directly to customers or restaurants. However, solar dehydration requires sunny days, lots of time, and space for drying. Space may be an issue for offshore farms with no associated land. This method also produces an inconsistent end product, making it unappealing for businesses looking to purchase seaweed and process it into a reliable consumer product.

Solar Chimney:

Solar chimneys are used to induce air to travel into the bottom of the chimney and rise past the drying product due to pressure and temperature differentials within the air column. This method requires slightly more capital and less space than simple air drying, but retains many of the same pros and cons. Solar chimneys could be an inexpensive and time-saving addition to an existing air-drying operation.

Unit Heater:

Using typical monthly wet and dry bulb temperatures for Fuqua Farms, the power required to heat a shipping container’s worth of air to 40, 50, and 70 degrees Celsius was calculated. Drying time at various temperatures followed an exponential decay curve, see Figure 1. Using the typical monthly temperatures, the drying time and power requirements were used to calculate the typical energy required to dry one batch of seaweed each month.

Unit heaters simply heat the air using electricity. A unit heater would be used to heat the air in the shipping container to dry out the seaweed. This would change the relative humidity while leaving the absolute humidity the same, increasing the moisture carrying capacity of the air. Moisture saturated heated air could escape the non-airtight container reducing the moisture content of the system. The unit heater system would use 906 kWh per year, well within the capacity of the solar panel system.

Figure 3: Unit Heater Power and Energy Requirement Charts

70 degrees Celsius is the recommended drying temperature, but the addition of a controls system could allow a temperature switch between 40 and 70 degrees in May and November to optimize energy use (see Figure 3).

Air Handling Unit

The air handling unit in this case would consist of an intake louver, centrifugal inline fan, electric heating coils, and a detached exhaust louver. The heated air will have a lower moisture content and higher moisture carrying capacity than the air inside the container. The fan will force air past the seaweed, accelerating the drying process compared to the unit heater’s natural air movement. The air will pick up moisture from the seaweed and then exit the container via the exhaust louver. This system’s power and energy requirements to dry one batch of seaweed each month are shown below, Figure 4. The air handling unit system would use 2487 kWh per year, within the capacity of the solar panel system. 

Figure 4: Air Handling Unit Power and Energy Requirement Charts

Dehumidifier

A dehumidifier is the most energy and capital cost intensive system analyzed here. Dehumidifiers use refrigerant gases to take air through the refrigeration cycle in order to pull moisture from it. Due to the ability to measure waste water volume and create an airtight environment, this method represents the highest degree of consistency and accuracy of the systems analyzed. However, the use of any refrigerant is problematic from an environmental standpoint, and the scale (size and cost) of machine required to dry these products may be prohibitive.

Comparison of Drying Methods by Cost

Capital cost basically covers buying the required equipment to run the system. For example, drying seaweed on the dock in the sun requires zero capital investment, while purchasing multiple dehumidifiers and solar panels requires significant capital investment.

Operating cost is calculated based on the relative energy cost of each system. This could also be called utility cost. The hope is that for any system this is negligible, as solar panels can cover the energy requirements for any of the drying methods explored.

Space cost is simply the physical space required for the system. Solar dehydration requires the most, air chimneys the next most. All other systems are assumed to exist within a standard shipping container for ease of comparison.

Time cost is the relative drying time for each system. Sun drying takes the most time, an air handling unit the least. This only becomes a limiting factor if the drying time is longer than the time between harvests, or if commercial concerns require a certain turnaround time.

Figure 5: Comparison Chart, Drying Methods

* As a result of the recommended solar system installation, operating cost or energy cost will be zero or negative as all systems’ energy requirements are within generation limits of the solar system. This means operating costs for these systems will actually be the capital cost of the solar system.

Recommendations

Recommendations for the optimal system are heavily dependent on each project’s unique requirements. For direct to consumer goods and small volumes, solar dehydration is clearly the best method. It requires almost no capital cost, no energy input, and has no environmental impacts. The downsides of this method are inconsistent results, extended drying times, and dependence on sunny days.

For farmers selling their products on to companies for further processing before reaching consumers, solar dehydration is simply not consistent enough. For these farms, an air handling unit system placed inside a shipping container is the best option. However, if product consistency is the priority, a dehumidifying system may be the best option. These are fairly energy intensive methods but, as proven in the calculations above, their energy requirements can be offset by the installation of a solar panel system on the roof of the shipping container and the waste heat from that system could even further accelerate the drying process inside.

Conclusions

Seaweed processing infrastructure is a key part of the overall cost of seaweed farming and can have a major impact on the feasibility of the overall business proposition. Infrastructure and energy costs are system dependent, and the optimal system is different for each project. Ultimately, while there is no one “right” answer, a careful assessment of the options is key to success. 

About the Author

Samantha Bernstein is a rising second-year MBA candidate at Duke University’s Fuqua School of Business. Prior to business school, she spent four years working as a engineer in the built environment, achieving licensure as a professional engineer. Samantha received her BSE in Mechanical Engineering from Duke University.