GB2356925A

GB2356925A - Control of heat loss through building envelopes

Info

Publication number: GB2356925A
Application number: GB9915735A
Authority: GB
Inventors: David Huw Stephens
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-07-06
Filing date: 1999-07-06
Publication date: 2001-06-06
Anticipated expiration: 2019-07-06
Also published as: GB2356925B; GB9915735D0

Abstract

Heat loss through a building envelope or element such as glazing is reduced by inducing external air to flow over the internal surface, thereby reducing the temperature difference between internal and external surfaces, such airflow providing the ventilation to the building. Stale air is extracted via a heat pump (5) which cools the air to below external temperature before rejection, extracting a net gain of heat to the building which is used to heat the incoming cold air before it reaches the occupied or functional parts of the building. Centrifugal fans (1) induce a flow of fresh air into ducts provided with slit apertures (2), allowing fresh air to discharge parallel with the underside of the sloping glass roof (3). The air is warmed while passing over finned pipes (4). Heat extracted from the outgoing air (6) by heat pumps (5) is used to warm the water not only in the pipes (4), but also that circulated in pipes (7) embedded in the soil.

Description

2356925 Control of heat loss through building envelopes

Background

This invention relates to ventilation and energy conservation in buildings.

The likelihood of causing climate change through increasing concentration of carbon dioxide in the atmosphere is compelling attention to reducing fuel consumption. That associated with buildings in the UK accounts for nearly 50% of emissions.

A substantial proportion of energy is consumed in making good heat loss through the envelope of the building, particularly through single glazing, which loses heat about three times faster than a solid brick wall.

The current technology for reducing heat loss is mainly improved thermal insulation, as by double or mulbple glazing, but heat loss remains relatively high. Thermal insulation of the opaque structure of a building is desirable, and is being implemented in new buildings, but it is expensive and unlikely to be cost effective on an existing building, unless there is a convenient cavity to fill.

Heat loss is proportional to the difference in temperature between inside and outside, such temperatures being the resultant of radiant heat transfer and conduction from the air.

According to the present invention, heat loss through an element of the building envelope is reduced by reducing the temperature difference across that element by causing incoming cold ventilating air to flow across that surface, thus distancing the warm internal air from such surface. Heat transfer by conduction from air to the inner surface of the element is reduced by the lower velocity of impact of gas molecules of the colder air on the surface than would be from warmer air. Heat transfer by long wave radiation from objects within the building would be little affected, but the consequent rise in temperature of the surface would partly recover that heat by transfer to the cold air curtain. Warm room air which mixes with the incoming cold air is carried back into the room.

This technique has most immediate application to glazing, including heated horticultural glass houses, but it can also be applied to walls and roofs of sheet metal, plastic, wood or masonry.

The utility of the method can be extended by providing a higher rate of flow of air than would otherwise be required just for ventilation, and using this air flow to feed a heat pump through which the air is exhausted, chilled and rejected. This abstracts renewable energy while the rejection temperature is below the ambient air temperature. This technique can be of particular value where buildings are difficult to air seal, from which there is a loss of heat by exfiltration of warm air. If extraction can maintain a negative pressure such heat loss will be reduced.

For comfort the incoming cold air should be heated at the base of the walls before it enters the living or working space, most appropriately with heat recovered by the heat pump from the outgoing air. A radiant heat component is desirable to compensate for the loss of radiant heat from the body to the cold surfaces.

A most advantageous application of this would be commercial glass houses, which suffer a high heat loss through the large area of glass roof. A high throughput of carbon dioxide is needed for photosynthesis, which if not supplied from burning fuel, must come from ventilation. At the same time, the relative humidity must be controlled to reduce risk of fungal disease. Yet the plants must be supplied with water, evaporation of which not only increases the relative humidity, but absorbs heat, which in winter would need to be supplied by extra fuel.

Moreover, electricity to drive the heat pumps could be supplied by cogeneration, the heat output being fully utilised towards heating the glasshouses.

The reduction of heat loss by a fresh air curtain, a plentiful rate of ventilation to supply carbon dioxide and control relative humidity, and recovery of not only latent heat of evaporation, but of liquid water which can be recycled for watering would seem to be a highly attractive application, but it has widespread potential for reducing heat loss from highly glazed offices, schools, etc.

Essential technical features Fresh air is admitted through adjustable apertures and constrained to flow across the inner surface of a building element substantially in a plane parallel to such element.

Commonly, in the case of a small vertical element such as a domestic window, the air inflow may be induced by the negative pressure in the building caused by extraction of air elsewhere, but such flow is affected by wind pressures.

Incoming cold air being denser than the warm internal air will tend naturally to flow downwards over the element.

Better control of air flow rates and direction can be achieved by fandriven air flows, and in such case the air flow may be contrived either to cling to the internal surface of the element by the Coanda effect, or to form a separate streamline flow which will maintain its integrity as a sheet over a useful distance.

Example

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which figure 1 shows a vertical section of a commercial horticultural glass house.

Centrifugal fans 1 induce a flow of fresh air into ducts provided with a slit aperture 2 within the ridge of the glass house, arranged to discharge fresh air parallel to the underside of the sloping glass roof 3.

Such incoming cold air is warmed by passing over finned pipes 4 before it reaches the level of the plants and is eventually exhausted through air-to-water heat pumps 5 being rejected as chilled air at 6.

The heat extracted from the outgoing air by heat pumps 5 is used to warm water, which may be circulated not only through pipes 4, but through pipes 7, which may be embedded in the soil, or beneath staging on which pots stand.

Figure 1 - Section of a commercial horticultural glass house fitted with fresh air curtain beneath the glazed roof, with heat pump heat recovery from exhaust air.

3 Control of Heat Loss through Building Envelopes

Claims

1. A method of reducing heat loss through a building envelope or element thereof whereby incoming ventilating air is constrained to flow over the internal surface of said envelope or element substantially parallel to said surface, such that the difference of temperature between air adjacent to the inside and outside surfaces is reduced.

2. A method as claimed in Claims 1 in which air is extracted elsewhere in the building and cooled by a heat pump to below external air temperature before being rejected, such that a net heat gain is extracted from the ventilating air flowing through the building and used to heat the building and particularly to heat the incoming cold fresh air before it reaches the occupied or functional region of a building.

3. A method substantially as herein described.

4. An application of the method to glasshouses and atria substantially as herein described and illustrated in the accompanying drawing.