Level 3 Environmental Technology Systems: the Unit 335 Guide

Electricity and heat, together, from one fuel. A micro-CHP unit runs an engine or a similar prime mover to generate electricity and captures the heat that would otherwise go up the flue, using it for the heating and hot water. Because the electricity is a by-product of running for heat, it only pays in a dwelling with a high space heating demand. It produces no hydrogen and no cooling.

Domestic micro-CHP units use a Stirling engine (the paper spells it Sterling), an external combustion engine driven by the burner's heat.

An evacuated tube collector seals the absorber inside a glass tube with the air pumped out. The vacuum removes conduction and convection losses almost entirely, so the collector holds its output in cold, windy weather and reaches higher temperatures than the other main type, the flat plate. The textbook describes a coated, double walled tube; the design guides describe a metal absorber, direct flow or heat pipe, sealed in a single tube. Finned, ribbed and ventilated tubes are not solar collectors at all.

A heat pump does not make heat, it moves it, on the same refrigeration cycle as a fridge: the evaporator absorbs, the compressor raises pressure and temperature, the condenser releases, and the expansion valve drops the pressure again. Dropping the pressure lowers the refrigerant’s boiling point below the ground temperature, which is the only way heat will flow into it. Nothing burns, so combustion is not involved.

An air source unit is a packaged outdoor box holding a fan, a heat exchanger acting as the evaporator, a compressor, an expansion valve, and a second heat exchanger that passes the heat into the system water. A ground loop collector belongs to ground source and a solar panel to solar thermal. A flue gas condenser is boiler equipment: a heat pump burns nothing, so it has no flue.

The main domestic biomass fuel is wood pellets, pressed from sawdust and shavings, with logs and wood chips as the alternatives. Biomass is called carbon neutral because growing timber absorbs the carbon dioxide that burning it releases. Kerosene and LPG are fossil fuels and smokeless ovoids are a coal product, so none of them is biological material.

Domestic micro-CHP units are gas fired and use a Stirling engine, an external combustion engine driven by gas expanding and contracting as it is alternately heated and cooled. The unit is heat led, generating only while the appliance is heating the house, typically 1 kW to 1.5 kW of electricity. Diesel, two stroke petrol and Wankel engines are internal combustion and are not used in these appliances.

A heat pump dislikes starting and stopping, and when underfloor zone valves close it can be left with almost no water to heat. A buffer vessel gives it somewhere to put surplus heat, and the volume to run a defrost cycle without robbing the house. A feed and expansion cistern is no help here, because a heat pump circuit is sealed rather than open vented.

Ground source has no combustion on site, no fuel to store, a long working life and relatively low maintenance requirements once the loop is in. Be careful with the other claims made for it. It does have moving parts, a compressor, circulators and a brine pump; it always needs ground works; and a horizontal collector needs a real area of land, so it will not fit any size of garden.

The loops in common UK use are vertical boreholes, horizontal trenches, the slinky and the pond loop, all closed loops of plastic pipe filled with water and antifreeze that pass collected heat to the evaporator. Strictly the slinky is not a fourth type but a way of configuring a horizontal trench, a coil that fits more pipe into each metre, so it saves excavation rather than area; the textbook and the trade still list it on its own. Aerial, radial, rooftop, cascade and reservoir are not collector types.