What happens when Scotland's wasted wind fills a Sunamp and a ZEB too

The peer-reviewed proof — what Edinburgh Napier and Worcester Bosch actually found

This article has a rare advantage: the concept it describes is not theoretical. It has been trialled in a real home, measured with calibrated telemetry, and the results were published at the International Retrofit Conference 2025 at the University of Salford. The paper — "Sunamp Heat Battery as a Pre-heating Solution for Domestic Hot Water Heating: A Case Study Evaluation" — was co-authored by Edinburgh Napier University, Worcester Bosch, Okana Global, and Sunamp.

The configuration tested is precisely what this article proposes scaling using curtailed wind energy: an electrically charged Sunamp pre-heater sitting in front of the gas boiler (or heat pump), fed by surplus renewable electricity via a diverter. In the study that electricity came from solar PV. In our proposal, it comes from Eddi Channel 2, absorbing curtailed Scottish wind that is currently being paid to not generate.

The 86% figure applies to summer months, when solar generation is consistently available. For our wasted wind application, the equivalent periods are high-wind events — which in Scotland are concentrated in autumn, winter and spring, and which correlate with the worst curtailment events. On a day like December 18, 2024, when Scotland's wind farms generated far more than the B6 boundary could carry south, every Sunamp on Eddi Channel 2 would be charging at maximum from free electricity. The emissions reduction during those events would approach the same 86% figure.

ePlus vs xPlus — why the distinction matters

It is worth being precise about which Sunamp product belongs in this system, because the two main Thermino variants work in fundamentally different ways and are not interchangeable for this application.

The ePlus has one further important characteristic for this application: it is future-proofed. Adding the Optimino ePV key at any point enables solar PV diversion as an additional charging source alongside or instead of the Eddi. A home that starts with an Eddi driving the ePlus from curtailed wind can later add solar panels and have both sources charging the same unit, maximising self-consumption across the whole year — solar in summer, curtailed wind in winter, cheap off-peak electricity as backup.

The dual-channel cascade — how it works in a real home

The system design follows a straightforward sequential priority. When curtailed Scottish wind creates surplus electricity on the local distribution network, the Eddi's CT clamp detects it and begins diverting in order of priority.

How it works differently for gas combis, system boilers, and heat pumps

Gas combi boiler homes — the majority of Scottish homes

Combi boilers have no hot water cylinder — they heat water on demand directly from the mains. This makes them incompatible with standard Eddi installations that target an immersion heater. The Sunamp ePlus on Eddi Channel 2 solves this elegantly: it effectively creates the storage the combi has never had, while simultaneously acting as a pre-heater. Cold mains water passes through the ePlus before the combi sees it. On a high-curtailment day, the ePlus is charged to 58°C and the water entering the combi may be at 45–50°C already — leaving the combi only a few degrees of work to do for DHW. The research result of 86% carbon reduction applies directly to this configuration.

System boiler homes with hot water cylinders

These homes have both a cylinder (Eddi Channel 1) and a boiler that heats the cylinder via a coil. The ePlus on Channel 2 adds a second layer: once the cylinder is fully charged by wind via Channel 1, the ePlus begins charging too. The result is a system where both hot water stores are full of free wind energy. The system boiler's role in DHW is almost entirely displaced on high-curtailment days, and gas consumption shifts primarily to space heating alone.

Heat pump homes

Heat pumps present the most technically significant efficiency gain from the ePlus configuration. A heat pump periodically switches from space heating to DHW heating — a high-temperature cycle that requires the heat pump to work at lower COP and often includes an immersion top-up to reach 60°C for Legionella. If the ePlus on Channel 2 provides pre-heated water before the heat pump's DHW circuit, the temperature rise required of the heat pump is dramatically reduced. On days when the ePlus is fully charged and can supply water at 55°C+, the heat pump's DHW immersion may not need to fire at all. The heat pump returns to space heating — the mode where it achieves its best seasonal COP — faster and more often.

The Tepeo ZEB — 40 kWh of space heating that charges itself from wasted wind

Everything so far has addressed domestic hot water. The Eddi on Channel 1 fills the cylinder. The Sunamp ePlus on Channel 2 pre-heats the water entering the boiler. But space heating — the larger of the two energy demands in most Scottish homes — has been handled by the gas boiler in our model throughout. That changes with the Tepeo ZEB.

Tepeo is a British clean-tech company founded in 2018 and manufactured in Wokingham, Berkshire. Its product — the Zero Emission Boiler, or ZEB — is exactly what its name suggests: a plug-and-play replacement for a fossil fuel boiler in a wet central heating system. It does not burn gas. It charges from electricity, stores heat in a high-density thermal core, and releases it on demand to the existing radiators or underfloor heating. From the household's perspective, the heating works identically to before. From the grid's perspective, it is a 40 kWh smart thermal battery that can be pointed directly at curtailment events.

Why the ZEB is already a wasted wind device — without needing an Eddi

The critical insight about the ZEB is that it does not require an Eddi to respond to curtailed wind. It does this natively. The ZEB's smart charging algorithm connects to time-of-use electricity tariffs — most importantly Octopus Agile, which prices electricity at half-hourly wholesale market rates. During curtailment events in Scotland, wholesale prices on Agile frequently fall to near zero or even go negative. The ZEB detects these prices, recognises them as the optimal charging window, and charges its 40 kWh thermal core at the cheapest, greenest, most wind-heavy moments on the grid — automatically, without any additional hardware.

This is the wasted wind absorption mechanism working as designed, at the consumer level. The ZEB is in effect a domestic-scale version of what grid operators are trying to achieve through Constraint Cost Programme and demand-side flexibility mechanisms — but it does it through market signals rather than direct grid instructions, requiring no new regulatory framework to function today.

The ZEB + Eddi + Sunamp system — the complete thermal cascade

When the ZEB is added to our existing Eddi and Sunamp system, the three products cover all of a home's thermal needs from curtailed wind electricity:

Proven, practical, scalable — and excluded from every government scheme

The most striking element of the Edinburgh Napier research is not the 86% figure. It is what the paper says about why this technology is not already in millions of homes. The researchers identified a comprehensive set of policy failures that together constitute a systemic exclusion of a proven low-carbon technology from the retrofit support ecosystem.

The policy exclusion is not just a problem for Sunamp. It is a problem for the wasted wind argument too. If Sunamp ePlus units were eligible for ECO4, a significant proportion of the 816,764 Scottish homes we are targeting could receive them at zero or low cost to the household. The constraint cost savings to the grid — currently £1.5 billion a year and rising toward £8 billion — would more than fund a national deployment programme. Instead, the government is paying to waste wind, while simultaneously refusing to fund the technology that would absorb it.

Scotland's three-product thermal fleet — the complete numbers