Biomass steam in both plans, PV on the factory roof only

User, 17 Oct 2026: "only put solar on the factory, prepare the other
buildings for solar, option a and b should operate on biomass".

- Steam: two 500 kg/h biomass modules (Getabec class, 6M to 10M THB) plus
  a fuel store and handling block (1M to 2M, estimate) in both plans; the
  electric bank becomes the alternative. ~0.67M THB of fuel a year against
  ~4.9M for electric steam; grid demand ~264 kW instead of ~980 kW.
- Solar: ~520 kWp on the factory roof (the biomass-case optimum is ~500 to
  550 kWp, hourly sketch in sustainability.md "PV size with biomass
  steam"); office roof and parking canopies built PV-ready (~120 kWp
  later). ~87 % used on site, ~61 % of the plant's electricity.
- Calculator: biomass default, bFuel row, pvKWp 520, pvSelf 87, iso fills
  the factory roof first and shows the other roofs as PV-ready.
- Numbers: launch plant ~168M / 232M / 296M with contingency; Plan A
  ~182M (~2M above 180M: the Chinese-route lever priced only electric
  boilers; a Chinese biomass quote is the way back), Plan B ~217M (label
  ~215M), ~35M more. Calculator EBITDA A 2.17M, B 2.23M USD.
- CSV, total-investment, revenue-model, sustainability, scale, biomass,
  site-candidates, boi, steam-cooling, construction RFQ (canopies
  PV-ready), README, roadmap; business plan draft 0.3 rebuilt (A and B
  63 pages) with new renders.

docs/sustainability.md

@@ -34,23 +34,25 @@ process cooling (up to ~650 kW), drawn in [steam and cooling](steam-cooling.html
   PEA's time-of-use tariff it earns little from peak shaving
   ([grid tariff](#grid-tariff-pea-and-what-it-decides)).
 - The machine load and cooling system are straightforward PV targets.
-- Steam is the open engineering question. Options under evaluation are an
-  electric boiler (maximizes the solar share) and a biomass boiler (locally
-  abundant fuel); solar-thermal pre-heating of boiler feedwater was
-  considered and is not recommended. No technology choice has been made
-  yet; it will be driven by honest numbers on load profiles, storage, and
-  grid backup, not by what sounds greenest. At PEA's tariff and current
-  northern fuel prices, biomass heat costs a quarter to a tenth of electric
-  steam
-  ([grid tariff](#grid-tariff-pea-and-what-it-decides)).
+- **Steam: biomass, decided 17 October 2026 for both business plan
+  versions** (user: "option a and b should operate on biomass"). At PEA's
+  tariff and current northern fuel prices biomass heat costs a quarter to
+  a tenth of electric steam
+  ([grid tariff](#grid-tariff-pea-and-what-it-decides)): for the launch
+  plant ~0.67M THB of fuel a year against ~4.9M THB for electric steam with
+  its demand charge (calculator, estimates). The electric bank stays the
+  alternative ([biomass](biomass.md)); solar-thermal pre-heating of boiler
+  feedwater was considered and is not recommended.
 - Whole-factory HVAC (continuous temperature, humidity, and PM control, see
   [facility systems](facility-security.md)) adds a major tropical-climate
   load on top of process cooling and must be part of the PV and battery
   sizing.
-- **Array size, decided 17 October 2026: every roof and canopy carries PV**
-  ("use as much roof space as possible"), so the kWp follows the site:
-  ~640 kWp on the Payap scenario plot, ~300 kWp on the Mueang Chi hall
-  (below). Storage beyond the 271 kWh ride-through battery stays open.
+- **Array size, decided 17 October 2026: PV on the factory roof only, the
+  other buildings PV-ready** ("only put solar on the factory, prepare the
+  other buildings for solar"): ~520 kWp on the Payap scenario plot,
+  ~247 kWp on the Mueang Chi hall; why that size is in
+  [PV size with biomass steam](#pv-size-with-biomass-steam). Storage beyond
+  the 271 kWh ride-through battery stays open.
 
 ### Roof design for PV (candidate Mueang Chi site)
 
@@ -111,28 +113,58 @@ the electric-boiler route is chosen.
 On the Payap scenario plot the 128 m factory roof takes ~901 modules
 (~523 kWp) and the car and scooter canopies another ~54 kWp, ~577 kWp in
 all ([site candidates](site-candidates.md#roof-pv-and-parking-canopies-part-and-draft-model-october-2026)).
-The flat office roof adds ~65 kWp around its three AC units (estimate
-from the [calculator](fleet-calculator.html) drawing), so covering every
-roof and canopy gives **~640 kWp**, the size the investment and both
-business plan versions carry since 17 October 2026. On the Mueang Chi
-hall the same rule gives only ~300 kWp (the 247 kWp roof above plus the
-canopies); more there means ground mount on the plot behind the building
-or the phase 2 roof.
+The flat office roof would add ~65 kWp around its three AC units (estimate
+from the [calculator](fleet-calculator.html) drawing), ~640 kWp with every
+roof and canopy covered. The plans carry **the factory roof only, ~520 kWp**,
+with the office roof and the canopies built PV-ready: structure rated for
+the panels (~20 kg/m² plus wind uplift), conduit and a cable route to the
+main board and a spare way for a later inverter, inside the building and
+site works (estimate). Their ~120 kWp go on when the load grows (the second
+Duplex system), for ~2.4M to 3.1M THB at today's prices. On the Mueang Chi
+hall the factory roof takes ~247 kWp and its canopies are PV-ready too.
 
-**What ~640 kWp means on the grid** (October 2026):
+### PV size with biomass steam
+
+With biomass steam the plant's running load is ~230 kW (no electric
+boilers), and a factory on PEA's grid cannot sell its surplus, so the array
+should not outgrow what the plant uses while the sun is up. An hourly
+sketch of a year (October 2026: two shifts on 300 days, ~60 kW off-shift
+base load, the 271 kWh battery, PEA TOU 3.2.2 with Ft, 23,000 THB/kWp
+installed, 400 THB/kWp a year upkeep, an inverter set in year 12, 25 years
+at 8 %; synthetic Chiang Mai sun profile, not PVGIS) gives, as estimates:
+
+| Array | Used on site | Saved a year | Payback | IRR | NPV | Payback of the last 50 kWp |
+|---|---|---|---|---|---|---|
+| 300 kWp | 94 % | 1.5M THB | 5.0 yr | 19 % | 7.1M | 5.4 yr |
+| 450 kWp | 90 % | 2.2M | 5.2 yr | 18 % | 9.7M | 6.0 yr |
+| 500 kWp | 88 % | 2.4M | 5.3 yr | 18 % | 10.2M | 7.1 yr |
+| 550 kWp | 85 % | 2.5M | 5.5 yr | 17 % | 10.3M | 9.0 yr |
+| 640 kWp | 78 % | 2.7M | 6.0 yr | 15 % | 9.8M | 14.6 yr |
+
+- **The best array is ~500 to 550 kWp, about the ~520 kWp factory roof.**
+  Above ~550 kWp each added kWp loses money: most of its yield is
+  curtailed on Sundays and at midday.
+- **The optimum follows the running load**, at ~2.3 kWp per kW: ~440 kWp
+  at 180 kW, the full ~640 kWp at 280 kW. Panel price, discount rate and
+  yield move it by only about 30 kWp. So the PV-ready roofs and canopies
+  are filled when the second Duplex system raises the load.
+- With electric steam (~430 kW running) every added kWp up to ~640 still
+  paid back in ~5.5 years, which is why the array was briefly sized to
+  every roof.
+
+**What ~520 kWp means on the grid** (October 2026):
 
 - **Zero export.** A factory on PEA's grid cannot sell its surplus: the
   interconnection needs a zero-export controller or reverse-power relay,
   and net billing is limited to small users
   ([CapSolar, rooftop solar regulations 2026](https://capsolar.co.th/en/knowledge/rooftop-solar-regulations-thailand)).
-  An hourly sketch (two shifts on 300 days, ~60 kW off-shift base load,
-  the 271 kWh battery; estimate) puts **~85 % of the ~900 MWh yield to
-  use on site**; the rest, mostly on Sundays and holidays, is curtailed.
-  With electric steam the launch plant draws ~2,000 MWh a year, so the
-  array covers ~38 % of it.
+  The sketch puts **~87 % of the ~730 MWh yield to use on site** (~633 MWh
+  in the calculator); the rest, mostly on Sundays and holidays, is
+  curtailed. With biomass steam the launch plant draws ~1,036 MWh of
+  electricity a year, so the array covers ~61 % of it.
 - **Permits.** Since Ministerial Regulation No. 3 B.E. 2567 (in force
   28 December 2024) rooftop solar outside industrial estates is not a
-  factory at any size, so no Ror.Ngor.4 licence; at ~500 kVA of inverters
+  factory at any size, so no Ror.Ngor.4 licence; at ~430 kVA of inverters
   the array stays under the ERC's 1,000 kVA line and needs an exemption
   notification, not a generation licence; PEA's interconnection review
   takes 3 to 6 months and is filed alongside procurement
@@ -151,8 +183,8 @@ or the phase 2 roof.
   at CapSolar ([CapSolar PPA](https://capsolar.co.th/en/services/ppa)).
   The calculator's 3.0 THB/kWh is ~21 % below the ~3.8 THB/kWh blended
   rate, inside CapSolar's range but above MEAei's usual discount (estimate,
-  to quote); the launch plant's bill (~0.75M THB a month before solar, calculator)
-  clears the threshold.
+  to quote); the launch plant's bill (~0.36M THB a month before solar with
+  biomass steam, calculator) clears the threshold.
 
 ### Energy efficiency architecture: recover before generating
 
@@ -295,10 +327,9 @@ not a cost one.
 (weekday daytime at the peak rate, weekends and early mornings at
 off-peak, Ft included), so the 247 kWp roof saves ~1.2M to 1.3M THB a year
 and pays back in **~3.8 to 5.4 years** at 20,000 to 26,000 THB/kWp
-installed. The full ~640 kWp array uses ~85 % of its yield on site, saves
-~2.8M to 3.0M THB a year and pays back in **~4.2 to 5.9 years**; the
-last ~340 kWp, more of whose yield is curtailed, take ~4.9 to 6.3 years
-(estimates; Thai market prices checked October 2026 in the
+installed. The ~520 kWp factory roof of the plans uses ~87 % of its yield
+on site with biomass steam, saves ~2.3M to 2.5M THB a year and pays back
+in **~4.1 to 5.8 years** (estimates; Thai market prices checked October 2026 in the
 [utilities CSV](finance/utilities-investment.csv): DEDE's PV status report
 gives 22 to 25 THB/W for factory systems of 10 kW to 1 MW in 2023,
 installers quote 18 to 25 THB/W in 2026). It does not lower the demand charge, because the peak window