13.5 Micro-Renewable Energy Technologies

13.5 Micro-Renewable Energy Technologies

Unit 203 learning outcome 6 (3 of the 40 test questions) asks you to describe the types of micro-renewable energy, identify the requirements for installing them (legal, regulatory and location), and identify their advantages and disadvantages. The range lists ten technologies: solar thermal (hot water), ground source heat pump, air source heat pump, biomass, solar photovoltaic, micro-wind, micro-hydro, micro-combined heat and power (heat led), rainwater harvesting and greywater re-use.

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Technologies That Produce Heat

Solar thermal (hot water)

Collectors on the roof (flat-plate or evacuated-tube) absorb heat from the sun. A pump circulates a water and glycol (antifreeze) solution through a sealed loop to a coil in a twin-coil hot water cylinder. A differential temperature controller runs the pump only when the collector is hotter than the cylinder. A boiler coil or immersion heater tops up the temperature when solar input is low. The electrician supplies the pump station and controller and wires the temperature sensors.

Heat pumps (air source and ground source)

A heat pump moves heat from a low-temperature source to the building using a vapour-compression refrigeration cycle:

1. Evaporator: cold refrigerant absorbs heat from the outside air or a ground loop and evaporates.
2. Compressor: an electrically driven compressor raises the refrigerant's pressure and temperature.
3. Condenser: the hot refrigerant gives up its heat to the heating water and condenses.
4. Expansion valve: the pressure drops and the cycle repeats.

The coefficient of performance (COP) is the heat delivered divided by the electrical energy used; typical values are about 3 to 4. Heat pumps work best with low flow temperatures, such as underfloor heating or larger radiators.

• Air source (ASHP): an outdoor unit extracts heat from the air. It is cheaper to install but less efficient in very cold weather, and the unit needs careful siting for noise and airflow.
• Ground source (GSHP): pipes in horizontal trenches (needing a large area of land) or vertical boreholes extract heat from the ground, which stays at a steadier temperature.

The electrician provides a dedicated circuit sized for the unit's running and starting current, with the protective device and RCD type the manufacturer specifies, and a local isolator.

Biomass

Boilers or stoves burn wood pellets, wood chips or logs. They need space for fuel storage and access for deliveries, a suitable flue, and regular ash removal. In smoke control areas only approved (exempt) appliances may be used. Biomass is treated as low carbon because new growth reabsorbs the carbon dioxide released.

Micro-CHP (heat led)

A micro combined heat and power unit, usually gas-fired, uses a small engine, Stirling engine or fuel cell to produce heat for the building and electricity as a by-product. "Heat led" means it runs when the building needs heat, so its electrical output follows the heating demand.

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Technologies That Produce Electricity

Solar photovoltaic (PV)

PV cells convert daylight directly into d.c. electricity. Panels are connected in strings, and an inverter converts the d.c. into a.c. synchronised with the supply. The installation needs d.c.-rated isolators, an a.c. isolator, a generation meter and warning labels, and must meet BS 7671 Section 712. Output depends on orientation (south-facing is best in the UK), tilt and shading. Battery storage is increasingly added; Amendment 4 introduced a new Chapter 57 for stationary secondary batteries.

Connection to the network: generators rated up to 16 A per phase connect under Engineering Recommendation G98, with the DNO notified within 28 days of commissioning. Larger systems need the DNO's approval before connection under G99.

Micro-wind

Small wind turbines on masts or buildings drive generators, usually through an inverter. They need a good average wind speed and smooth airflow; turbines on buildings in towns often perform poorly because of turbulence. Planning, noise, vibration and mast structure all need considering.

Micro-hydro

A turbine driven by flowing water generates electricity continuously where a suitable stream or river has enough head (height difference) and flow. Schemes usually need permissions such as an abstraction licence from the environmental regulator, and protection for fish and wildlife.

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Technologies That Save Water

Rainwater harvesting

Rainwater from roofs is filtered into a storage tank (often underground), and a pump supplies non-drinking uses such as WC flushing, washing machines and garden taps. A mains top-up feeds the tank through an air gap so that drinking water cannot be contaminated. The electrician wires the pump and its control panel.

Greywater re-use

Lightly used water from baths, showers and basins is collected, filtered or treated, and stored for WC flushing or garden use. It must not be stored for long because bacteria can grow, and the pipework must be clearly marked. The electrician wires the pumps, treatment units and controls.

Installation Requirements

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Advantages and Disadvantages

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Related Low-Carbon Installations

• Battery storage: stores surplus PV output for use later; installed to BS 7671 Chapter 57 (new in Amendment 4) and the manufacturer's instructions.
• EV charging points (BS 7671 Section 722): on a PME supply an outdoor charge point needs measures against a broken PEN conductor, such as an open-PEN protection device, a TT earth electrode or an isolating transformer, and the right RCD protection (for example a Type A RCD with a 6 mA d.c. detecting device, or a Type B RCD). Amendment 4 also recognises RDC protective devices for EV charging.