6 Strategies for Improving the Total Efficiency of Marine Biorefineries
6.1 Smart Mixing
Systems to Improve
Total Productivity
One of the limiting factors in the macroalgae cultivation in the
food, chemicals, and fuels value chain is the biomass yields. One
of the limitations of the biomass yield is the ability of photosynthetic receptors to capture photons and regenerate, a process
known as light/dark reactions [52]. Plants have evolved the ability
to harvest almost 100% of the arriving protons; however, the total
photosynthetic efficiency is only around 5% due to the low utilization of photons. In the normal illumination, at the outdoor environment, the rate of photosynthesis is not limited by a number of
photons. Multiple studies have shown that the rate of photosynthesis is limited by multiple physiological processes such as diffusion,
plant metabolism, and carbon fixating reaction metabolism
[53–57]. Studies have shown that equal photosynthetic rates can
be achieved by continuous and pulsed light (with specific frequencies) [58]. This property has been widely used in the design of the
onshore photobioreactors, when mixing was used to intensify the
total microalgae and macroalgae yields [59, 60]. In this process,
however, mixing is usually used to improve the nutrients diffusion
and provide optimum aeration to the plants. In such mixing
regimes, most of the energy is wasted on the friction with walls.
A new concept for an offshore macroalgae cultivation system in
which an increase of yields per area could be achieved by external
mixing, adapted to the natural photon capture:carbon fixing rates
ratio, was introduced. In this mode, “smart” mixing enables utilization of the volume of the floating offshore macroalgae reactor, by
exposing the macroalgae cells to the solar energy for a short time to
capture photons and then to take the algae to the depth for a period
of time required for carbon fixation, when a new layer of algae is
exposed to the sun. The ultimate goal of this system is to increase
the total energy efficiency of the biorefinery increasing the photon
utilization yields per area of installed macroalgae offshore farm.
In the offshore environment, pulsed light effect can be
achieved by macroalgae mixing when the thallus (with the characteristic dimension l) is exposed to the light only for a limited time
(t light ) to capture photons. For the rest of the time in the cycle
(t dark ), the thallus can be in the shadow to complete the carbon
fixing reaction and recovering the photosynthesis center absorbent
capacity. The schematic representation of the simplest macroalgae
mixing system (in two-dimension) in the offshore condition is
shown in Fig. 6. Different from the onshore tubular and flat photobioreactors, which can absorb light from different directions, the
photon flux in the offshore conditions comes only from the water
surface. The length of the path the single thallus is making under
the ideal mixing conditions is:
Design and Analysis of Offshore Macroalgae Biorefineries
23
6.1 Smart Mixing
Systems to Improve
Total Productivity
One of the limiting factors in the macroalgae cultivation in the
food, chemicals, and fuels value chain is the biomass yields. One
of the limitations of the biomass yield is the ability of photosynthetic receptors to capture photons and regenerate, a process
known as light/dark reactions [52]. Plants have evolved the ability
to harvest almost 100% of the arriving protons; however, the total
photosynthetic efficiency is only around 5% due to the low utilization of photons. In the normal illumination, at the outdoor environment, the rate of photosynthesis is not limited by a number of
photons. Multiple studies have shown that the rate of photosynthesis is limited by multiple physiological processes such as diffusion,
plant metabolism, and carbon fixating reaction metabolism
[53–57]. Studies have shown that equal photosynthetic rates can
be achieved by continuous and pulsed light (with specific frequencies) [58]. This property has been widely used in the design of the
onshore photobioreactors, when mixing was used to intensify the
total microalgae and macroalgae yields [59, 60]. In this process,
however, mixing is usually used to improve the nutrients diffusion
and provide optimum aeration to the plants. In such mixing
regimes, most of the energy is wasted on the friction with walls.
A new concept for an offshore macroalgae cultivation system in
which an increase of yields per area could be achieved by external
mixing, adapted to the natural photon capture:carbon fixing rates
ratio, was introduced. In this mode, “smart” mixing enables utilization of the volume of the floating offshore macroalgae reactor, by
exposing the macroalgae cells to the solar energy for a short time to
capture photons and then to take the algae to the depth for a period
of time required for carbon fixation, when a new layer of algae is
exposed to the sun. The ultimate goal of this system is to increase
the total energy efficiency of the biorefinery increasing the photon
utilization yields per area of installed macroalgae offshore farm.
In the offshore environment, pulsed light effect can be
achieved by macroalgae mixing when the thallus (with the characteristic dimension l) is exposed to the light only for a limited time
(t light ) to capture photons. For the rest of the time in the cycle
(t dark ), the thallus can be in the shadow to complete the carbon
fixing reaction and recovering the photosynthesis center absorbent
capacity. The schematic representation of the simplest macroalgae
mixing system (in two-dimension) in the offshore condition is
shown in Fig. 6. Different from the onshore tubular and flat photobioreactors, which can absorb light from different directions, the
photon flux in the offshore conditions comes only from the water
surface. The length of the path the single thallus is making under
the ideal mixing conditions is:
Design and Analysis of Offshore Macroalgae Biorefineries
23