Active solar space heating

Adding pumps or fans to passive solar water or air heating systems can greatly increase their overall efficiency and sophistication, at the expense of greater capital and running costs and lower reliability. Active solar space heating systems can take full advantage of thermal store possibilities to significantly reduce the need for back-up space heating. More solar heat can be utilised for longer periods, and extremes of temperature can be mitigated more effectively.

Active solar water space heating is not yet a fully established technology, compared to solar-heated DHW. The first generation of solar water heating systems were usually retrofitted to individual dwellings, which normally had limited potential for really significant reduction of their space heating needs through upgraded insulation, etc. Practical considerations limited the size of the hot water storage that could be retrofitted, and hence the actual solar collector area used was relatively small; only a fraction of the roof area, for example. And for most installations the temperature of the water in the thermal store during cold weather was lower than that at which conventional radiators are efficient. In such situations it made perfect sense to use solar heating for DHW only - and even then a conventional back-up heater was invariably installed as well. Space heating relied on other technologies, from woodburning stoves to gas-fired boilers.

As low energy construction technology developed, however, the potential to use solar-heated water for space heating in new buildings increased. Improved solar collectors (see Chapter 9) and cost-effective large thermal stores (see Chapter 15) made it possible to utilise a much higher fraction of the sun's energy to heat water to higher temperatures. This increased energy availability could be used to either increase the proportion of DHW heated by solar gain to 100% right through the year or to provide a significant proportion of both DHW and space heating demand. Using lower temperature underfloor heating pipes rather than conventional wall-mounted radiators is also a practical option on new buildings.

Even with well-insulated buildings that make maximum use of passive solar gain, heating enough water to provide both DHW and space heating will require a significantly larger solar collector area. Planning and/or aesthetic considerations would normally dictate the choice of either some form of solar roof, solar slates or tiles or facade-integrated

Heat Undfloor Heating
Underfloor heating is suitable for the largest buildings (Reproduced with permission from Nu-Heat UK)

collectors when solar water space heating is to be provided. Where visual intrusion is not an issue, a roof-mounted array of conventional flat plate or evacuated tube collectors might be preferred. The closer to true south the collectors face the better; the usual advice is to tilt them to latitude plus 15° to maximise heat gain during winter.

Tall buildings have a relatively small roof area and a high surface to volume ratio. In situations where current or future shading from adjacent buildings is judged to be no problem, facade-integrated collectors would be the obvious choice, and could almost certainly capture enough solar energy to provide a very high fraction of the hot water needed for both DHW and space heating. Tall buildings often have deep basements, where a large thermal store could be conveniently located.

Solar Heat Capture For Winter
Fagade-integrated collectors could be the best choice for solar water heating on tall buildings (Reproduced courtesy of Solarnor)

Active direct solar water systems (see Chapter 9) can be used for space heating. The heat transfer fluid circulates through the collectors, into an insulated thermal store, then through the radiators or underfloor heating network. A secondary loop allows back-up heating as needed. This system could be kept completely separate from the solar DHW heating system, or they could be integrated, with the DHW heated by some form of heat exchanger in the space heating storage tank. Some systems, often known as solar com-bisystems, take elaborate measures to promote stratification within a common thermal store. DHW is drawn off from the upper, hotter part of the tank; space heating uses the cooler water lower down. A completely separate space heating system has the advantage that the collectors can be drained down in summer.

However, active indirect systems are the normal choice. A heat transfer fluid carries energy from the collectors to a high temperature thermal store. DHW and space heating water pass through independent heat exchangers in the thermal store. Stratification is often utilised as well. Several pumps and a sophisticated electronic controller are needed, and most thermal stores will normally have to be positioned at ground or basement level because of their weight. At the time of writing, it is unlikely that a conventional thermal store large enough to provide more than 50% of both DHW and space heating needs throughout the year would be economic for an individual dwelling, even if space could be found for it. On the larger scale, however, the calculations could well be more positive. And there is the added bonus that all elements of such a system are well developed, and that a number of specialist manufacturers in Europe and North America can supply modular packages offering a choice of technologies, including warm air heating.

Solar Air Heating Systems
Solar space heating system using solar-heated water/air heat exchanger

Modern low energy buildings usually need constant ventilation to maintain internal air quality, as they are deliberately made as airtight as possible to prevent warm internal air leaking out through the building envelope. In terms of energy efficiency, however, allowing warm stale air to leave the building through a simple ventilator is no better than losing it through gaps around windows and doors. Some form of heat recovery ventilation (HRV) or energy (or enthalpy) recovery ventilation (ERV) is the obvious solution, and many proprietary systems are available. HRV technology is based on straightforward heat exchanger principles that extract heat from the outgoing stale air and transfer it to the incoming cold air. ERV units also transfer humidity from outgoing to incoming air, maintaining internal humidity at comfortable levels.

Fresh air to house

Stale air from house

Circulation fan ^

Controls

Heat-exchange core

Casing

Interior of the home

Circulation fan

Trap filled with water

Outdoors Screen

Controls

Heat-exchange core

Casing

Interior of the home

Fresh air to house

Stale air from house

Circulation fan ^

Fresh Air Intake House Fan

Hood

Fresh air intake

Screen

Circulation fan

Trap filled with water

Exhaust air outlet

Hood

Fresh air intake

Screen

Principles of a heat recovery ventilator

Air entering an HRV or ERV system must be above freezing point or there is a risk of ice forming. One way round this - used in Europe - is to draw the air first through earth ducts. These are smooth large diameter plastic or metal pipes buried at least 1 m below ground level, where temperatures rarely drop below 7°C. Such systems can also work 'in reverse' in summer, to pre-cool incoming air and reduce its humidity (see Chapter 8). Regular maintenance is required, especially for ERV units. Mains power is almost universally used.

Heat that would otherwise go to waste can be recovered from a number of sources. Domestic hot water often goes down the drain only a little cooler than when it left the thermal store. Passing it through a heat exchanger before it goes to waste is an obvious option. The recovered heat might be returned to the diurnal thermal store directly, or be used to preheat mains water entering the diurnal thermal store, or go to a seasonal thermal store outside the building.

There are some very simple ways of improving occupant comfort by recirculating warm air. One of the most basic just recirculates the warmer air from attics, with the intake immediately under the peak of the pitched roof. This air has normally been heated by a combination of solar gain and heat rising from the rooms below, and is normally no more than a supplement to another form of space heating system. Only effective where the loft insulation is immediately above the ceiling of the rooms below - and the underside of the roof proper is uninsulated - the system's efficiency can be increased by the installation of very simple collectors on the underside of the roof. This is particularly effective when the roof is dark-coloured; natural slate is a prime example.

Low Pitched Loft Attic Space
A simple heat exchanger recovers useful energy from waste DHW

A further development is the Nuaire Sunwarm system, which adds roof-mounted TAPs (see Chapter 3). The hot air from the TAPs and the attic space is circulated through ducts for space heating and also provides DHW via a roof-mounted heat exchanger.

Sunwarm Nuaire
The Sunwarm warm air space heating system uses heat from TAPs and the attic, and can also provide DHW (Reproduced with permission from Nuaire)

Sometimes this heated air is circulated through a gravel bed diurnal thermal store or a low temperature seasonal thermal store under the ground floor (see Chapter 15). Even simpler is a hot air recirculating system for occupied areas with no ceilings and roofs at high level - basically a high-efficiency fan is fitted below roof level to collect the rising hot air and blow it straight downwards at relatively high velocity, back to occupant level. Air below a well-insulated roof is typically 5°C warmer than at occupant level, so significant savings on space heating costs can be achieved.

Section Monodraught
Monodraught's Heat Harvester recirculates warm air from under the roof (Reproduced with permission from Monodraught)

Adding fans and ductwork to a basic thermal or Trombe wall (see Chapter 3) greatly increases its potential efficiency. Heated air can be distributed to all sections of the building

Thermosyphoning
Thermosyphoning air panels offer a space heating solution for light industrial buildings (Reproduced with permission from SolarVenti)

and discharged into the occupied areas at relatively low velocity through large vents, to maximise occupant comfort. Where traditional construction is preferred for dwellings, forcing air heated by a Trombe wall through the crawlspace between the ceiling and the floor above converts the mass of the floor construction into a reasonably effective thermal store that will increase efficiency and prolong the comfort period.

Thermosyphoning air panels are often connected to the intake of a conventional air heater by means of ducts and fans. Another alternative is the roof space solar energy collector, which is essentially a pitched roof partially or fully glazed on its southern aspect. Heated air can either be distributed into the building directly or be fed to a warm air space heating system. This can be a very cost-effective option, as the preheating of the feed air can significantly reduce the energy consumption of the air heater, however it is fuelled.

Products of combustion to atmosphere

Insulated opaque Foul air to partition atmosphere \

Damper to ventilate RSC to gable ventilators

Transparent cladding

Products of combustion to atmosphere

Insulated opaque Foul air to partition atmosphere \

Damper to ventilate RSC to gable ventilators

Transparent cladding

Roof Space Collectors

Ventilation through eaves

Typical roof space solar energy collector space heating and ventilation system

Ventilation through eaves

Typical roof space solar energy collector space heating and ventilation system

On the larger scale, facade-integrated collectors consisting of plastic coated steel or aluminium cladding panels with around 2,500 minute perforations in every square metre are available in a wide range of colours. These, sometimes known as transpired air collectors, are normally mounted 100 mm away from the internal wall, and in their darker colour versions are said to be able to raise the temperature of the outside air entering the cavity through the perforations by more than 20°C. In the summer they act as a sunscreen, with the heated air discharged back to the atmosphere at roof level. Solar PV panels are often used to power the fans in this type of installation.

Transpired Air
Transpired air collectors work well with solar PV powered fans (Reproduced with permission from CA Gro)

Simple thermal stores for solar air heating systems normally utilise gravel, and are not particularly efficient compared to water-based stores (see Chapter 15). However, in some applications - such as low-rise schools and offices - their basic simplicity and low capital and running costs can make them very attractive. The most common form is the gravel bed store, supplied by TAPs; an option that is, however, only open to new build projects. An insulated waterproof box immediately below a concrete ground floor slab is filled with clean, dry 'single size' gravel that contains little fine material that could obstruct airflow through the stones. Air from TAPs or other forms of solar air collectors is circulated through the gravel bed.

This type of installation has to be backed up by some other system of space heating, but it has the advantage of working quite efficiently with relatively low temperature air from the collectors as none of this enters the living space. The crawlspace under a suspended concrete or timber ground floor can also act as a thermal store; timber is a 50% more effective thermal store than gravel, and the ground itself can also make a significant contribution.

TAPs are available as a package with a fan driven by an integral solar PV panel. This form of space heating, particularly when combined with a rock bin or underfloor gravel bed, is much more common in the US than in Europe, for example, but is well worth considering for smaller projects. In many cases, the arguments in favour of solar-powered space heating are closely linked to the increased need for summertime cooling (see Chapter 13). Using the same solar collectors to power both heating and cooling will make a lot of sense on many projects.

Transpired Solar Collectors
Buildings with façades made up of transpired air collectors look little different to conventional structures (Reproduced with permission from CA Group)

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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Responses

  • innes mackay
    Why fresh air intake humidity low in room?
    7 years ago

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