Displacing Electrical Load - Grid Stability - Technical Annex

8 Annex Quantifying Electrical Load Displacement

TECHNICAL ANNEX A

Quantifying Electrical Load Displacement

A Worked Proof-of-Concept Model for a Representative Dense Mixed-Use District, and a Carbon-Accounting Challenge to the “Low-Carbon Only” Heat-Network Presumption

Companion to: Position Paper — Displacing Electrical Load.   Status: Draft proof-of-concept for consortium and funder discussion

A.1  Purpose and Status of This Annex

This annex serves the two objectives identified for the IUM programme. First, it provides a worked proof-of-concept: a transparent, order-of-magnitude engineering model that puts numbers on the central claim of the position paper — that bio-methane CCHP with absorption cooling displaces electrical load from a stressed grid while returning synchronous stability to the point of demand. Second, it constructs a carbon-accounting argument that challenges the prevailing policy presumption that heat networks must, in effect, be electrically-driven to be considered low-carbon.

A.2  The Representative District

To keep the model concrete, it is built around a single representative district of the high-density, mixed-use type the IUM Garden City is designed around — the Canary Wharf typology, where heat and cooling loads are sufficiently concentrated for district energy to be optimal.

The cooling load is the focus here because it is the largest discretionary electrical load in a commercial district and the one most directly coincident with summer grid stress.

A.3  The Two Routes Compared

A.3.1  Conventional electrical air conditioning

Meeting the 16 MW cooling peak with conventional electric chillers at a seasonal energy-efficiency ratio (EER) of 3.5 draws electrical power directly from the grid:

• Peak electrical demand: 6 MWe of peak grid demand, occurring on hot, often low-wind afternoons when the system is least able to supply it; and
• Annual electricity: 3,200 MWh per year of grid electricity, with no contribution of any kind to system stability.

A.3.2  Bio-methane CCHP with absorption cooling

In the CCHP route, the cooling is produced thermally. A bio-methane engine set generates electricity, and its recovered waste heat drives double-effect absorption chillers (COP ≈ 1.1). To meet the 16 MW cooling peak, the absorption plant requires approximately 14.5 MWth of recovered heat, which sizes the engine set as follows (delivered as multiple units plus thermal storage for resilience and diurnal balancing):

The same fuel that produces the cooling also produces roughly 14 MWe of electricity from a synchronous alternator — electricity that serves the district and, at the margin, exports to or relieves the local grid.

A.4  Headline Result: Quantified Grid Relief

The grid impact of the two routes is not merely different in degree; it is opposite in sign. The electrical route adds load; the CCHP route removes load and adds local generation.

A.5  The Stability Contribution — What Electric AC Cannot Provide

Beyond load, the CCHP route returns the synchronous services whose loss is destabilising the grid. The engine set (≈ 16 MVA at 0.9 power factor, inertia constant H ≈ 1.5 MW·s/MVA) provides:

• Real inertia: approximately 24 MW·s of stored rotational kinetic energy — genuine physical inertia that resists frequency excursions instantaneously, without software mediation;
• Voltage support: local reactive-power capability for voltage support, delivered at the point of demand where voltage — unlike frequency — must be locally maintained; and
• Fault current: strong, instantaneous fault current that protection systems can detect — a service inverter-based plant can only imitate by expensive and uneconomic over-sizing.

Electrical air conditioning provides none of these. It is, in stability terms, pure unpredictable power-electronic load on a system that is losing its ability to carry such load safely.

A.6  The Carbon-Accounting Challenge to DESNZ

The second objective of this annex is to challenge the policy presumption — widely read into the heat-network framework — that low-carbon heat must mean electrically-driven heat. The actual regulatory position is more specific than a blanket prohibition, and that specificity is where the IUM model finds its opening.

A.6.1  What the framework actually requires

Under heat-network zoning and the emerging technical standards, government designates zones where heat networks are expected to be the lowest-cost route to decarbonising heat, and a carbon limit applies from 2030 with a phased tightening thereafter. Crucially, the carbon-accounting methodology already contains an explicit credit for electricity generated by CHP — in the consultation methodology, a factor of 304 gCO₂/kWhe of generated electricity. In other words, the framework already concedes the central principle of the IUM argument: that on-site generation which displaces grid electricity should be credited against the carbon of the heat.

A.6.2  The genuine, live dispute — and why bio-methane resolves it

The substantive criticism of this methodology — made by CIBSE among others — is not that the CHP credit principle is wrong, but that the 304 gCO₂/kWhe factor is far too generous to fossil gas CHP: it is roughly double the realistic grid marginal carbon factor, and the grid exceeds that level for only a few dozen hours a year. The fear is that the methodology rewards gas CHP for displacing a dirtier grid than actually exists, and will exist even less in future.

This critique is valid against fossil-gas CHP. It does not apply to bio-methane CCHP — and that distinction is the heart of the IUM case:

1. The objection to gas CHP is about the carbon of the fuel and the over-generous electricity credit. Bio-methane carries a near-zero biogenic combustion factor (conservatively ≈ 22 gCO₂/kWh against ≈ 183 gCO₂/kWh for natural gas), so the heat itself is low-carbon at source — independent of any electricity credit.
2. The displacement benefit is real and conservative. Because bio-CCHP both removes cooling load and supplies synchronous generation, it relieves the grid by some 18.8 MWe at peak in the worked example — a benefit that holds even if the electricity credit factor is revised down to the realistic grid marginal figure.
3. The framework’s own logic supports it. The methodology already credits CHP electricity; the only honest correction is to use a defensible carbon factor and a genuinely low-carbon fuel. Bio-CCHP satisfies both — it is precisely the case the credit was designed to recognise, done honestly.

A.7  Carbon Routes at a Glance

Only the bio-CCHP route scores well on all three dimensions at once. Electrification addresses fuel carbon only by transferring the burden onto a grid that cannot safely carry it; fossil-gas CHP relieves the grid but fails the carbon test; bio-CCHP does both.

A.8  Assumptions, Limitations and the Funded Workstream

In the interest of the intellectual honesty that makes the case persuasive, the principal simplifications are stated openly:

• Indicative parameters: Cooling intensity (80 W/m²), full-load hours (700), chiller EER (3.5), engine efficiencies (42% / 43%) and absorption COP (1.1) are representative mid-range values; site-specific figures will differ.
• Steady-state sizing: The model sizes the energy centre to the cooling peak; a real design balances heat, cooling and power across the year with thermal storage, and would optimise engine count and dispatch accordingly.
• Carbon factors: Carbon factors are conservative point values; the funded workstream will use half-hourly grid marginal data and a full bio-methane lifecycle figure including upstream and digestate credits.
• Economics: Capital and operating cost, and the consumer-bill comparison that the regulation ultimately turns on, are deliberately out of scope here and form the next deliverable.

The funded modelling workstream will convert this proof-of-concept into a validated, site-specific model for a candidate Garden City district — producing the quantified avoided peak load, returned inertia, local voltage support and lifecycle carbon figures needed to take the IUM case from proposition to demonstrated evidence, and to engage DESNZ, Ofgem and the relevant combined authority on the basis of numbers rather than assertion.

Source note: Regulatory framing draws on DESNZ heat-network zoning materials and the Warm Homes Plan (2024–2026), the Heat Network Technical Assurance Scheme consultation (2026), and the CIBSE consultation response identifying the 304 gCO₂/kWhe CHP electricity credit. Grid-stability framing follows Kathryn Porter, Watt-Logic (2026). Engineering values are standard mid-range estimates for UK district energy and are indicative only.