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Technical answers - pricing microregeneration, building temperatures and bacteria in ductworkJune 2010

David Bleicher provides the answers to tricky technical questions posed by BSRIA Members. Problems solved this month include electricity charges, bacterial counts and shading coefficiency.

Microgeneration costs

Q. I have heard that owners of microgeneration systems will soon be paid for the electricity they produce. How does this work for stand-alone (not grid-connected) systems?

Microgeneration sample feed-in tariffs

The government proposes to implement feed-in tariffs from April 2010. Anyone who generates electricity using small-scale low carbon technologies will receive a guaranteed payment for all electricity they generate, called the generation tariff (see sample tariffs in the table below). In addition, they will receive a payment for all electricity they export to the grid, otherwise known as the export tariff.

The government's consultation proposes that the generation tariff should vary depending on the type and scale of technology used to generate electricity, but the export tariff should be fixed at 5p/kWh. More details on the scheme, including the proposed generation tariffs, can be found by going to the DECC website then downloading the document Renewable Electricity Financial Incentives Consultation Reponses.

By way of example, a stand-alone photovoltaic or wind generator system would receive a generation tariff but not an export tariff, as it would be incapable of feeding in to the grid.

Building temperature

Q. Are there any minimum and maximum temperatures for buildings?

The Fuel and Electricity (Heating) (Control) (Amendment) Order 1980, 1013 stated that the maximum heating temperature in office buildings should be 19°C. Although this was intended as an energy saving measure, it has never been enforced in practice.

The Workplace (Health, Safety and Welfare) Regulations 1992 place a legal duty on employers to maintain a reasonable temperature in their workplaces. The minimum temperature is normally considered to be 16°C, however this could vary depending on the level of activity and other requirements, such as food safety.

There is no widely accepted maximum temperature. Further guidance can be found in HSE Approved Code of Practice L24. Table 3.1 in CIBSE Guide B1. CIBSE makes recommendations for internal design temperatures in various types of buildings, but these should not be considered as minimum or maximum values.

Solar heat gain

Q. What does the abbreviation SC stand for?

SC is often used as the abbreviation for shading coefficient. Shading coefficient is the solar heat gain coefficient of a glazing system divided by the solar heat gain coefficient for a clear single pane of the same size.

Alternatively, a shading coefficient can be the ratio of solar heat-gain through a glazing system (under a specific condition) to the solar gain through a single pane of a reference glass (double-strength sheet glass with 0.86 transmittance, 0.08 reflectance, and 0.06 absorptance at normal incidence) under the same conditions.

This coefficient can vary with changing solar spectral distributions, especially if the glazing system to which it applies is strongly spectrally selective. It can also vary with direction of incidence.
ASHRAE recommends a spectral-based method for calculation of the solar heat-gain coefficient, giving the fraction of incident-beam solar radiation transmitted by a window for the specific solar spectrum incident on the window and for the specific direction of incidence.

Hence a shading coefficient would equal the solar heat gain of the glazing system divided by the solar heat gain of standard reference glass.

For direct solar beam radiation having a standard spectral distribution perpendicular to the glass, the shading coefficient is the solar heat gain coefficient divided by 0.87. It would have a value of 1.0 for clear single pane glass with the specifications given above, and a solar heat gain coefficient value of 0.87. (That said, this is not exactly true for other angles of incidence, nor for non-standard solar spectral distributions.)

CIBSE's TM26:2000 Hygienic maintenance of office ventilation ductwork guide is available for members to borrow from the BSRIA library

Ductwork surfaces

Q. What are reasonable limits for bacterial counts in samples taken from ductwork surfaces?

The limits for bacterial counts are given in CIBSE TM26: 2000 Hygienic Maintenance of Office Ventilation Ductwork.

TM26 states: "the action level for recommending cleaning of the ductwork due to microbial problems should be set at >29 colony forming units per 10 cm2" (Paragraph 10.2.1). This publication also provides the protocol for surface sampling.


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