While various parts of the system certainly could be improved to give better overall efficiency, that is not the focus of this project. Rather, we are looking at how existing technologies can be linked together in a specific context to make economic and environmental sense. In the future, solar technology must move out of the labs and into real-life applications, and the best way to do so is to take a holistic look at the resources available in a specific area. The novelty of this system is not the parts, but the connections between them that make the system a viable choice.

We did not include the sale of the xerogel in to the equation due to the fact we could not find a reliable cost for the material. Most values that we were coming across were for small scale quantities and to scale up those values to the quantity of product we would be producing, we felt, would skew our reported value to a lower cost than we would actually be able to live up to. At $0.34 kWh we are getting close to being competitive with fossil fuels, but not enough to make an impact. Adding the sale of the xerogel into the equation would have been a beneficial facet as it would have lowered our reported value and made our end result more competitive with the current markets. 
In addition, our preliminary estimates show that the volume of xerogel that could be produced from rice hulls would be large compared to the current market, raising the possibility that implementing GREEN RICE could reduce the market price of xerogel. This is a factor we plan to explore more in future research

1) “Your plant requires 144 tons/day of rice husks. What is the energetic/financial cost of collecting that supply from fields and then transportation to your plant?”

This is something that we haven’t looked deep into, however, our idea is that the plant would be located in the vicinity of the rice fields so that the additional transportation of the rice husks would be minimal compared to how they would be moved anyway.

2) “What cost is CO2 separation & cooling from noxious exhaust gases before delivery to algae?”

We don’t expect that cooling would be necessary, as a large portion of the heat will be taken by the steam generator and the heat transfer from the bubbling gas to the liquid is not significant enough. Additionally we could choose a thermal tolerant algae or cyanobacterium.

There isn’t any need to separate the CO2 from the flue gas because it will dissolve into the media as the algae consume it.

In terms of flue gas pretreatment, the algae liquid will act as somewhat of a scrubber collecting fly ash and dissolving some constituents which could be recycled to the rice field. What may cause some concern would be toxicity (of metals or sulfur), but at this point we don’t believe there would be an accumulation of anything from the rice husks that would cause a significant problem.

3) “Do you recover your processing and capital costs for SiO2 and algae end products, including transportation to consumers (e.g., rice fields) to justify these activities?”

Our preliminary analysis of the economics of the system suggests that the system would recover the processing and capital costs, though the revenue stream is dominated by xerogel sales rather than avoided fertilizer costs from algae end products. Put differently, while there are environmental benefits associated with capturing the carbon dioxide from the generator and reducing the need for energy-intensive fertilizer production, these alone do not justify the economics, hence our focus on high-value end products such as xerogel. The current market price of xerogel, how our system could affect the xerogel price by increasing supply, the broader environmental implications of deploying xerogel for energy efficiency purposes, and the detailed costs of the system are factors we plan to explore more in future research.

4) “Finally, how big and how expensive(capital costs; not including daily water, heating, and fertilizer costs) would be an algae farm that could process 90% of the CO2 produced from a 144 ton/day rice combuster (likely producing ca. 300 tons CO2/day)?”

The algae farm would only be able to absorb CO2 during the day, so it would only receive flue gas half the time, but we approximated a foot print of about 2,000,000 square meters and a volume of 50,000,000 L. We estimate that the capital cost of the photobioreactor will be approximately $100,000,000, though there is considerable uncertainty about the cost and we plan to iterate this estimate based on more detailed designs of the system. Some of the key uncertainties are economies of scale, the extent to which we will be able to leverage existing photobioreactor designs, and the lifetime of the system/the need for replacing parts. To put this in perspective, the capital cost is equivalent to about five years’ worth of revenue for a rice farm of the size under consideration, so financing and subsidy considerations were central to our analysis, as well. We also have sought to understand the non-economic benefits, as well, though so far we have only done a detailed analysis of carbon dioxide emission reduction as a proxy for sustainability benefits in general.

Thanks for your detailed reply. Good luck with your project!

An example of a scientific challenge may be that there is literature on growing algae on flue gas, but this usually targets coal flue gas which may pose completely different challenges. We presume that there would be less issue with biomass flue gas however, this is an unknown. Choosing an algae or cyanobacterial strain that will tolerate the conditions that we have while showing high growth rates and producing a valued product is a challenge. We believe Anabaena is a good candidate however it may not grow well or there may be opportunities for higher value product. We discussed the possibility of using genetically modified organisms, where we would have the photobioreactor produce a fuel. Our design has been built around a system where all the parts are located in close proximity which would have a potential for leaks to the immediate ecosystem and GMO’s aren’t considered environmentally friendly and have regulatory issues to deal with, as well, leading us to discard this idea.

As for economic challenges, our system would be seeking to compete against existing sources of electricity, and as such we have examined what role subsidies could/should play in making the system more competitive. Due to the need for competitiveness, we have put major emphasis on finding high-value commodities such as xerogel that could be produced by the system and sold in order to make the system economically sustainable. Additionally, financing is a major consideration as the system would take at least a few years to pay for its own capital costs, so we have looked at the economic and non-economic considerations that might go into a rice farm’s decision to adopt or not adopt such a technology at their facility.

The true breakthrough for our project is the integration of the diverse and proven technologies (i.e. biomass combustion to fuel, uptake and assimilation of CO2 by a photobioreactor, production of fertilizer by introducing biomass from the photobioreactor to the rice field and processing of rice husk ash to produce a pure silica product).

We specifically stayed away from GMO’s as we were trying to use only proven technologies that could be implemented directly to a thriving eco-system without having an adverse or detrimental impact. The photobioreactor is still a newer technology (proven in small-scale lab settings) and we foresee that we may have some technical issues to work out when scaling up to the level that we would need to offset our carbon emissions from combustion.

Additionally, the production of xerogel from silica contained initially in rice husks is the most novel portion of this project. The current starting materials for silica xerogels are expensive, hazardous to a persons health and highly flammable. Currently the resulting ash is treated as a low value product, but after processing we will obtain a product that has a high value and is commercially relevant.

Rice is currently stored in the hull until an order comes in for it, at which point it is milled, so there is already a natural storage component in the system.
Thank you for your interest!

Currently rice husks can be used in the brewing industry, in gardening, and for animal bedding. The rice husk ash can be used as an additive to cement, as a soil ameliorant, oil absorbent, in lightweight construction materials, silicon chips, water purification, vulcanising rubber and ceramics. Not all of these application are regularly used. There is a fair amount of research going into uses since the husk is a large waste product for many rice farmers.