Replacing a coal boiler in a glasshouse tomato operation
A 2.4 ha glasshouse near Tuakau produces truss tomatoes at 52 kg/m² under contract at $4.30/kg. Heat and CO2 come from a 1.7 MW coal boiler burning 340 t/yr at $470/t; coal emits 2.4 t CO2e per tonne. Flue gas is scrubbed and used to enrich the glasshouse to 750 ppm, which lifts yield by an estimated 12% relative to ambient. Carbon is priced at $65/t CO2e and electricity at $0.17/kWh. Three options are on the table: a biomass boiler at $850,000 capital, woodchip at $85/t using about 480 t/yr, plus $20,000/yr extra maintenance and a possible air discharge consent; an electric heat pump with purchased CO2 at $1.5 million capital, 0.75 GWh/yr extra electricity, and liquid CO2 at $0.38/kg for about 270 t/yr; or dropping enrichment and running at ambient CO2. The crop is also exposed to tomato brown rugose fruit virus, and the site records about 40 contractor and visitor movements a week.
1. Compare the three options on annual cost, emissions and risk, and recommend one.
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Current position. Coal: 340 t × $470 = $159,800. Emissions: 340 × 2.4 = 816 t CO2e × $65 = $53,040. Total $212,840/yr.
Option 1 - biomass boiler. Fuel 480 t × $85 = $40,800, plus $20,000 maintenance = $60,800/yr. Saves about $152,000/yr, so payback on $850,000 is roughly 5.6 years. Emissions are biogenic and generally treated as low, and the flue still supplies CO2 for enrichment. Risks: air discharge consent for particulates and NOx, fuel quality and security of woodchip supply, higher boiler supervision, and the biomass is not free of stack emissions even if the carbon is counted as biogenic.
Option 2 - heat pump with purchased CO2. Electricity $0.17 × 0.75 GWh = $127,500, plus CO2 270 t × $0.38/kg = $102,600, total $230,100/yr. Slightly more than the current $212,840, and $1.5 million of capital, so on today's carbon price it is not financially superior. It is, however, the lowest-emission option, avoids a stack entirely, and improves by about $8,200/yr for every $10/t rise in the carbon price. At $120/t the coal option costs $257,720, so the heat pump becomes cheaper to run.
Option 3 - ambient CO2. Yield falls 12% of 52 kg/m² × 24,000 m² = about 150 t, worth $644,000 at $4.30/kg. Removing CO2 is by far the most expensive option, not the cheapest, because the CO2 purchase cost ($102,600) is a small fraction of the yield it protects.
Recommendation. Biomass now, provided the air discharge consent is obtainable and a woodchip supply contract can be secured. It captures most of the savings at lower capital and preserves CO2 enrichment. Hold the heat pump as the next investment, triggered by a carbon price above about $110/t or by the boiler reaching end of life.
2. Explain why dropping CO2 enrichment is not a cost-saving measure, and identify the conditions under which the 12% yield response would be overstated.
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Why it is not a saving. The 12% figure is an annual average response to raising CO2 from ambient to 750 ppm. At 52 kg/m² over 24,000 m², output is 1,248 t; 12% is about 150 t, worth roughly $644,000 at $4.30/kg. The cost of supplying that CO2, whether from a boiler flue or purchased at $102,600, is a small fraction of the value of the crop it supports. Removing enrichment would save the CO2 cost but lose the yield, and the fixed costs of glasshouse, labour and packing would be spread over 12% less fruit. It would also make it harder to meet contract volume.
When the 12% would be overstated.
- Seasonal timing: enrichment only works when the glasshouse is closed. In summer, when vents open for temperature control, added CO2 escapes and the marginal response is near zero. A significant part of the annual response comes from winter and spring, when light may then become limiting.
- Light: below a light threshold, extra CO2 does not translate into extra growth. In a low-light winter, the response is smaller than the average.
- Temperature and humidity: if the crop is being run cool to save energy, the CO2 response falls.
- Crop load and variety: the response assumes the crop is not already limited by truss load, fruit set or nutrition.
- The 12% may be a vendor or industry figure measured on a different system. It should be validated on this site by comparing blocks over a season.
Practical implication. Do not treat the 12% as a constant. Budget the decision using seasonal enrichment, running high CO2 only when the house is closed and light is adequate. That also lowers the annual CO2 requirement, which improves the heat pump case.
3. Tomato brown rugose fruit virus reaches the region. What biosecurity measures would you put in place, given 40 contractor and visitor movements a week?
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ToBRFV is mechanically transmitted and extremely stable on surfaces, tools, crates, clothing and in plant debris and irrigation water. It is also seed-borne at low rates. With 40 movements a week, the site's biggest risk is people and equipment, not insects.
Entry control.
- A single controlled entry point with a signed log, mandatory change of footwear or boot wash, hand washing and disposable overalls for visitors and contractors.
- No entry to any growing house for anyone who has been in another tomato or capsicum operation that day, or the previous day. That rule is inconvenient and must be enforced by the owner, not delegated.
- Dedicated tools, knives and harvest equipment per house, disinfected between uses. Shared crates and pallets are a known route; use own or confirmed clean crates.
Plant and seed.
- Buy seed and seedlings only from suppliers who test for ToBRFV, with certification and batch records retained.
- Propagate in a separate, biosecure nursery area with its own staff and equipment.
Water and waste.
- Disinfect recirculated irrigation water by UV or slow sand filtration.
- Remove and destroy suspect plants in sealed bags; do not compost on site. Clean and disinfect the area afterwards.
Monitoring and response.
- Weekly scouting for mosaic, mottling and fruit bronzing; laboratory testing of any suspect plant.
- A written response plan: who is notified, how the block is isolated, what is destroyed, how long before replanting.
Contractor management. Induct every contractor once, in writing, and make access conditional on compliance. Bumblebee supplies should come from a reputable source, since bees and hives can also move virus. The cost of these measures is small relative to losing the crop.