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Bioengineering / Microalgae

Algae Photobioreactor Productivity Simulator

Compare the specific growth rate, annual biomass, lipid yield, CO₂ fixation and CAPEX of major microalgae — Chlorella, Spirulina, Nannochloropsis and Haematococcus — across Raceway, tubular, flat-panel and airlift photobioreactors. A productivity sandbox for algal biofuel and food / cosmetic ingredient design.

Parameters
Algal species
Sets μ_max, half-saturation K_I, T_opt and lipid content
Reactor type
Sets light-utilisation factor and CAPEX per m³
Light intensity (PAR)
μmol/m²/s
Photosynthetically active radiation at the surface. Outdoor noon ~ 2000
Culture temperature
°C
Cell density X
g/L
Dry-weight biomass concentration in the culture
Reactor volume
L
Lab 1 L to commercial 1,000,000 L (1000 m³)
CO₂ supply
%
Air 0.04%, coal flue gas 12–15%
Results
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Growth rate μ (1/day)
—
Vol. productivity P_v (g/L/day)
—
Annual biomass (kg/y)
—
Lipid yield (kg/y)
—
CO₂ fixation (kg-CO₂/y)
—
CAPEX (USD)
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Reactor cross-section — light, CO₂ and algal cells

Green dots are algal cells, yellow rays from the top indicate incoming PAR, white bubbles from the bottom are sparged CO₂, and the horizontal arrow shows liquid circulation. Cell density and light intensity drive the visible density and beam length.

Growth rate vs light (Steele/Monod saturation)
Reactor type — volumetric productivity
Theory & Key Formulas

$$\mu = \mu_{max}\,\frac{I}{K_I + I}\,f(T),\quad P_v = \mu \cdot X,\quad P_a = P_v \cdot h$$

μ = specific growth rate (1/day), I = light intensity (μmol/m²/s), K_I = half-saturation constant, X = cell density (g/L), h = reactor depth. f(T) is the Cardinal Temperature Model with a triangular profile between T_min, T_opt and T_max.

Algae Photobioreactor Productivity — Biofuel and Food Design

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A photobioreactor (PBR) is basically a tank for growing algae, right? What is really different from a normal greenhouse for plants?
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The principle is the same — light goes in, carbon is fixed — but the target is a single-celled microalga, so the productivity per area jumps by an order of magnitude. A corn field gives 20 to 30 tonnes of biomass per hectare per year, but a Spirulina raceway can hit 50 to 80 t/ha and a closed Nannochloropsis system has been reported above 100 t/ha. And because there is no root or stem to throw away, 25 to 40 percent of the dry mass becomes lipid you can turn into biodiesel, EPA, DHA, astaxanthin or Spirulina protein.
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There are four species in the dropdown — Chlorella, Spirulina, Nannochloropsis and Haematococcus. Why are the numbers so different from one to the next?
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Each one targets a different product. Chlorella vulgaris is the workhorse: μ_max = 1.2 /day, about 25 percent lipid, good for biodiesel feedstock. Spirulina grows slowly (μ_max = 0.6) but loves 35 °C and pH 10, which keeps competitors out, and it is more than 60 percent protein — that is why the global Spirulina food supply is dominated by open raceway ponds. Nannochloropsis is 40 percent lipid and rich in EPA for aquaculture and human nutrition. Haematococcus pluvialis is the slow one (μ_max = 0.4) but stores up to 4 percent astaxanthin under high-light stress, fuelling the cosmetic and supplement market. Cyanotech, AlgaTech, DSM and Solazyme are the historical names.
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If I switch the reactor to Raceway the light-utilisation factor drops to 0.40 and productivity falls. So why are real Spirulina plants almost always raceway ponds?
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CAPEX. A raceway is about 200 USD per m³, a tubular PBR 800, and a flat-panel 1200. Because Spirulina is run at pH 10, the open pond barely sees contamination, so the cheapest reactor wins. For high-value products like astaxanthin or EPA, where the kilogram price is in the tens to hundreds of dollars, the four-fold extra CAPEX of a closed reactor pays back through purity and yield. Try comparing the annual biomass and CAPEX in this tool — the cost per kg ratio tells you almost immediately which reactor class fits each product.
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When I push the light slider from 100 up to 2500, μ keeps rising but the curve bends a lot. Why does it saturate?
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It is the Steele/Monod light saturation. Once I exceeds the half-saturation K_I (around 200 μmol/m²/s for Chlorella), K_I + I is dominated by I, so μ approaches μ_max asymptotically. Above roughly 1000 μmol you also see photoinhibition: the PSII reaction centre is damaged faster than it is repaired and net photosynthesis actually drops. That is why outdoor cultures often grow slower at noon. Designers mitigate this by mixing the cells through light–dark cycles (flashing-light effect), thinning the culture, or accepting Beer–Lambert self-shading at depth.
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The CO₂ fixation number says a 1000 L reactor can capture roughly 118,000 kg of CO₂ per year. Is algae really a carbon-negative biofuel?
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On paper yes — 1.83 kg of CO₂ per kg of dry biomass from the elemental composition CH1.8O0.5N0.2. So 65 tonnes of biomass per year captures around 119 tonnes of CO₂. The catch is the life-cycle analysis: raceway paddle-wheels, water makeup, harvesting, dewatering and lipid extraction all need energy, which today is largely fossil. Algenol, Sapphire Energy and others tried to close that loop at scale during the 2010s and could not make the economics work. The current realistic path, used by ChitoseGroup and Euglena Corp, is to sell high-value food and cosmetic products, and treat CO₂ capture as a co-benefit rather than the main revenue stream.

FAQ

A Raceway pond costs about 200 USD/m³ in CAPEX, which is roughly a quarter of a tubular PBR at 800 USD/m³, and scales easily to thousands of square metres for commodity products like Spirulina food. The trade-off is a low light-utilisation factor (~40%) and exposure to rain, evaporation and biological contamination. A tubular PBR with ~85% light use and a closed loop is preferred for high-value products such as Haematococcus astaxanthin or Nannochloropsis EPA, where the higher CAPEX is justified by the price per kg. This tool compares both volumetric productivity and CAPEX side by side.
Chlorella vulgaris has a high maximum specific growth rate (μ_max ≈ 1.2 /day) and about 25% lipid content, which makes it attractive as a biodiesel feedstock. Spirulina (Arthrospira) grows more slowly (μ_max ≈ 0.6 /day) but its optimum at 35 °C with alkaline pH ~10 strongly suppresses contaminants, and its >60% protein content has made it the most widely produced microalgal food worldwide. Nannochloropsis is a 40% lipid, EPA-rich source for aquaculture and nutrition, while Haematococcus accumulates ~4% astaxanthin under high-light stress for cosmetic and supplement use.
No. The Steele/Monod model μ = μ_max·I/(K_I+I) saturates once I exceeds the half-saturation constant K_I (about 200 μmol/m²/s for Chlorella). Beyond about 1000 μmol/m²/s, photoinhibition damages PSII reaction centres and growth actually drops. Practical PBR design therefore aims for an effective I roughly 3 to 5 times K_I and uses mixing, dilution and Beer–Lambert based self-shading to spread the light through the culture depth. The growth-vs-light chart in this tool shows the saturation directly.
Conditionally yes. Stoichiometrically, microalgae fix about 1.83 kg of CO₂ per kg of dry biomass, and combustion releases the same amount that was captured, giving a carbon-neutral cycle on paper. With sustainable electricity, recycled water and reuse of de-oiled residue as feed or fertiliser, a life-cycle analysis can reach net-negative. In practice, however, the energy needed for mixing, water make-up, dewatering and solvent extraction is often supplied by fossil fuels. Large-scale biofuel ventures such as Algenol and Sapphire Energy could not close the economics during the 2010s. The current consensus is to monetise high-value food, cosmetic and aquafeed products first, with fuels as a by-product.

Real-world applications

Food and supplements (Spirulina, Chlorella): Global Spirulina production exceeds 10,000 tonnes per year, dominated by Cyanotech (Hawaii), Earthrise (California) and DIC Lifetec (Japan/Thailand/USA). Chlorella has been produced for over 50 years at scale by Sun Chlorella and Yaeyama Shokuhin (Okinawa) for protein, chlorophyll and probiotic-substrate uses. Running this tool at 1000 m³ reactor volume quickly shows why production sites need 1 to several hundred hectares of pond area.

Functional lipids (Nannochloropsis EPA, Schizochytrium DHA): Plant-derived long-chain omega-3 fatty acids that replace fish oil. Nannochloropsis stores ~40% lipid, of which >30% is EPA, and is commercialised by DSM, Cellana and Qualitas Health. Multiplying the lipid output of this tool by the EPA share gives a quick yield estimate. DHA from Schizochytrium (heterotrophic labyrinthulids) feeds vegetarian and infant-formula markets under DSM's life'sDHA brand.

Astaxanthin cosmetics and supplements (Haematococcus pluvialis): A red carotenoid accumulated under high-light stress, with strong antioxidant properties and a price in the 1,000–10,000 USD/kg range. AlgaTech (Israel), Cyanotech and Fuji Chemical Industries share the world market. Because Haematococcus has μ_max = 0.4 (slowest in this tool), CAPEX of 800 to 1200 USD/m³ for a closed tubular or flat-panel reactor is essentially mandatory — the simulator makes this trade-off obvious.

Carbon capture, wastewater and aquafeed: Direct injection of power-plant flue gas (12–15% CO₂) into PBRs is being demonstrated as Carbon Capture and Utilisation (CCU) by IHI, Mitsubishi Heavy Industries and Algenol. The CO₂ output of this tool (stoichiometric 1.83 kg-CO₂ per kg-biomass) lets you compare the algal mass needed against a power plant's emissions. Wastewater applications consume nitrogen and phosphorus from municipal effluent as nutrients (NEDO, Kyoto University, ChitoseGroup MATSURI project).

Common pitfalls

The first trap is assuming a lab μ_max carries directly to an outdoor pond. The μ_max values in this tool (1.2 /day for Chlorella, etc.) are measured under continuous light, constant temperature and optimal pH. Outdoors, light is only available for ~12 h per day and the culture respires away 5–15% of the biomass at night. Diurnal temperature swings frequently push f(T) below 0.5. Real plant productivity is typically 30–50% of the theoretical value shown here. Treat the simulator output as the upper bound of what the species can do, not the median.

The second trap is ignoring self-shading and photoinhibition when choosing a light intensity. This tool uses an averaged I_avg = I × lightUtil for simplicity, but real cultures follow a Beer–Lambert profile I(z) = I₀·exp(−ε·X·z). In a 30 cm deep raceway at 5 g/L the bottom layer receives less than 1% of the surface light, so most cells run in dark respiration. At the top, intensities above 2000 μmol/m²/s damage PSII through photoinhibition. The art of PBR design is using mixing — Taylor–Couette flow, airlift downcomers, static mixers — to expose every cell to a flashing light cycle.

Finally, do not equate lipid mass with biodiesel yield. The lipid_pct used here is total cellular lipid, which sums polar membrane lipids and triacylglycerol (TAG). Only the TAG fraction is biodiesel feedstock and is typically 50–70% of total lipid in Chlorella. Nitrogen starvation pushes TAG up to 50–60% of dry weight, but division halts and μ falls toward zero, so productivity (g-lipid/L/day) peaks under moderate conditions — the classical two-stage cultivation strategy. Headlines claiming "50% lipid achieved" usually describe conditions where overall productivity is poor.

How to Use

  1. Enter photosynthetically active radiation (PAR) in μmol/(m²·s), typically 150–800 for outdoor cultivation and 200–400 for indoor systems
  2. Set culture temperature in °C: most microalgae (Chlorella, Scenedesmus, Nannochloropsis) optimize at 20–28°C; temperature drift beyond ±3°C reduces growth rate by 8–15%
  3. Input cell dry weight density in g/L; typical inoculum ranges 0.5–2.0 g/L for batch mode, up to 5–8 g/L in fed-batch
  4. Specify reactor working volume in liters; simulator calculates volumetric productivity, annual yield, lipid extraction mass, and CO₂ sequestration across 330-day production cycles

Worked Example

Chlorella vulgaris in 100 L tubular photobioreactor: PAR = 400 μmol/(m²·s), 24°C culture temperature, 1.8 g/L initial cell density. Simulator outputs: specific growth rate μ = 0.62 day⁻¹, volumetric productivity P_v = 1.14 g/(L·day), annual biomass = 37.6 kg/year, lipid yield = 7.5 kg/year (20% extractable lipid), CO₂ fixation = 67.2 kg-CO₂/year (stoichiometric 1.88 kg CO₂ per kg biomass), CAPEX = USD 8,400 (including aeration, LED/solar coupling, temperature control, monitoring).

Practical Notes

  1. Nannochloropsis and Botryococcus demand higher PAR (500–600 μmol/(m²·s)) for maximum lipid accumulation; Chlorella tolerates 200–300 with acceptable yield
  2. Dissolved oxygen saturation above 200% inhibits growth; ensure aeration flow 0.5–1.0 vvm (volumes per volume per minute) in closed systems to prevent photoinhibition
  3. CAPEX scales nonlinearly: 10 L bench system ~USD 1,500; 1,000 L pilot ~USD 35,000; outdoor ponds 10,000+ L drop to USD 2–3 per liter but sacrifice control and contamination risk rises
  4. CO₂ injection at 2–5% concentration and 0.1–0.3 L/(L·min) boosts growth 20–35%; direct flue-gas coupling reduces inlet cost by 40% but requires pH buffering (sodium bicarbonate, 0.5–1.5 g/L)