Add energy efficiency architecture: heat recovery before generation

Press cooling return (60 C) feeds heat pump into boiler feedwater; waste
heat runs label oven and HVAC dehumid reheat; solar thermal preheat;
steam accumulator; boiler does only the final lift. Heat pumps for the
final 180 C steam and ground-source in tropical Thailand both ruled out
with reasoning; northern Thailand LPG tank context documented

docs/sustainability.md

@@ -42,6 +42,53 @@ at 10 bar / 180 °C) and the 250 kW cooling system.
 - System sizing (kWp, storage, grid-connection model) is open until the site
   is chosen.
 
+### Energy efficiency architecture: recover before generating
+
+The deepest efficiency in this plant comes from its own thermodynamics: the
+steam's job is to heat the molds, and the cooling water's job is to remove
+that same heat again, so roughly 98% of the 250 kW cooling load is heat
+leaving the press molds, put there almost entirely by the steam, and the
+press cooling return exits at ~60 °C. That
+warm return is a premium heat source, and the architecture is built around
+reusing it before generating anything new:
+
+- **Feedwater heat recovery**: a water-to-water heat pump lifts the 60 °C
+  press cooling return into boiler feedwater at 90 to 95 °C. Each kWh it
+  consumes does double duty: it cuts the boiler's work and removes load from
+  the chiller at the same time. Together with condensate return, this saves
+  roughly 10 to 15% of steam energy while shrinking the cooling system.
+- **Waste heat for low-grade consumers**: the label dry oven and HVAC
+  dehumidification reheat (the 35 to 60% RH spec means air is cooled below
+  dew point and reheated continuously) run on recovered press heat instead
+  of fresh electricity.
+- **Solar thermal feedwater preheat**: flat-plate or evacuated-tube
+  collectors reach 60 to 90 °C trivially in the Thai sun; cheap, proven, and
+  stackable with the heat pump recovery.
+- **Steam accumulator**: the presses draw steam in bursts, so an accumulator
+  (already a Newbilt option) lets a smaller boiler ride the peaks.
+- **The boiler only does the final lift**: whichever technology is chosen
+  (electric, LPG, or biomass), it supplies just the last step to 180 °C /
+  10 bar from already-hot feedwater.
+
+Two honest exclusions, checked and ruled out:
+
+- **Heat pumps cannot make the final steam**: commercial steam-generating
+  heat pumps top out around 120 to 165 °C at lower pressures than the fixed
+  Newbilt spec of 10 bar / 180 °C; hybrid heat-pump-plus-compression systems
+  exist as industrial pilots, but core utility reliability is the wrong
+  place to pioneer.
+- **Ground-source (earth temperature) does not work in tropical Thailand**:
+  the ground sits at 28 to 30 °C year-round, far too cold to be process heat
+  and warmer than the 20 to 25 °C cooling loop wants, so boreholes help
+  neither side. That budget is better spent on PV panels.
+
+Fuel-side context for the boiler choice: there is no natural gas pipeline in
+northern Thailand, so gas firing means an on-site bulk LPG tank (roughly 2.5
+to 5 tonnes, with hazardous-storage separation distances and licensing), and
+diesel means a fuel tank with price exposure. The electric and biomass paths
+avoid a fossil tank on the property entirely, which matters for both the
+sustainability commitments and the BOI story.
+
 ## Zero contaminated water
 
 The reference process schematic shows a conventional open "dirty water"