As I’ve posted previously:

A direct gain system is one where the main means of thermal gain is through direct heating of the thermal mass by solar radiation entering the building through windows and being absorbed by the thermal mass. An indirect gain system is one where solar radiation is captured and stored in a component of the building that has a high thermal mass and from there released to areas within the building that require heating.

That’s all well and good, but what does this mean in practical terms, how do we design our buildings to maximise solar heating? Here are a few tips:

Direct Passive Solar Heating

• You need plenty of double or triple glazed energy efficient glass that will let the heat in and keep it in. If possible plan for glazing the equivalent of up to 25% of the floor space. Windows should face as close to due south as possible, although anywhere within 30 degrees of this will still provide solar gain.
• To allow a degree of control over the amount of sunlight entering the building all windows will need curtains, shutters or blinds that can be used to shut out excessive sun-light.
• Don’t cover stone or concrete floors with carpets, they will inhibit solar energy from reaching and being absorbed by your thermal mass.
• Dark coloured materials will absorb more thermal energy than light ones.
• When considering building materials keep the word mass in mind – the more the merrier.
• To shelter especially exposed elevations, consider planting deciduous trees that will bear leaves and create shade over the summer and help prevent overheating. In the winter the trees will shed the leaves and let what sunshine there is through.

Indirect Passive Solar Heating

Perhaps not well suited to our temperate UK climate were days on uninterrupted sunshine are few and far between, I find the inventiveness of indirect passive solar heating solutions very attractive. For example, the Trombe wall or window box.

To build a Trombe wall. (also known as a solar window), take a south (or north in the southern hemisphere) facing 8 inch or so thick masonry wall, paint it black on the outside then glaze leaving an inch wide gap between the wall and the glass. Sunlight will hit the wall, be absorbed into and radiate through the wall to heat the interior. Heat travels through a masonry wall at about an inch an hour so as the interior cools in the late afternoon, heat that was absorbed by the wall earlier in the day will radiate into and warm the room beyond the wall.

A variation on this is a Window box passive solar heating system. For one of these, build a box beneath a window, glaze it and fill it with stones. Sunlight will heat the stones, heated air will rise up in the box and pass into the building through a vent that passes from the top of the box, through the wall underneath the window and into the interior of the building to be heated. A door over the vent can be used to regulate the interior heating.

Isolated Gain Passive Solar Heating

Not really one for the UK but in sunnier locals sun-spaces (also known as a solar room or solarium) are areas of a home, usually central to the layout, with lots of vertical glazing, minimal horizontal glazing and plenty of thermal mass. Doors into the rooms that lead off the sun-space are used to regulate the passage of heat. Read more at energysavers.gov

For anyone interested in indirect passive solar heating solutions then I thoroughly recommend, The Barefoot Architect which has plenty of suggestions accompanied by some (occasionally indistinct, but nevertheless useful) pictures.

One of the cornerstones of energy efficient, healthy, sustainable building design, the PassivHaus standard aims to provide comfortable year round living conditions through minimal energy expenditure.

These aims are achieved through:

• An effective passive solar design that will provide the necessary heat gain (heating).
• To manage the heat gain:
• …very highly specified insulation,
• …near complete airtightness.
• Mechanical ventilation coupled with highly efficient heat recovery and ‘backup’ heating systems to manage the internal climate.

No additional heating systems are required.

The PassivHaus standard for buildings in Europe dictates that the building will consume no more energy than:

• Heating & Cooling: 15kWh per m2 floor area per annum.
• Total Primary Energy* Consumption: 120kWh per m2 floor area per annum for all appliances, domestic hot water and heating and cooling.

* Primary Energy is drawn from the national grid which is inherently inefficient, much energy being lost during distribution.

passivhaus.org.uk

Visit the BRE managed PassivHaus site for further details of the specification.

A term often heard, but of rarely understood, just what does it mean?

Some definitions:

“within ten years every new home will be a zero-carbon home“

Gordon Brown, then Chancellor, in pre-budget report 2006.

When asked for further clarification, a zero-carbon home was defined as one that does not contribute to global warming - hardly a precise definition.

The zero carbon building produces no Carbon Dioxide and by combining all the available innovations can actually export carbon free energy back into the electricity grid.

http://www.applied-energy.com/ definition

Definitions can tend to ignore the CO2 emissions related to the sourcing of materials and the construction of the fabric of a building and in the initial provision of services and supporting infrastructure. A genuinely zero carbon building must be able to payback the carbon invested in its construction through generating and exporting zero carbon energy back into the national grid.

So where does that leave us. Personally, I am happy to use the catch-all zero carbon as an umbrella term incorporating both practical, real world low carbon design and an aspiration toward a truly zero carbon building lifecycle.

A zero carbon house is one that maintains ongoing CO2 emissions as near to zero as possible. The zero is currently aspirational.

Is the negative carbon house somewhere around the corner?

One of those frequently occurring eco-building terms, thermal mass is often mentioned but rarely defined.

The thermal mass of a building is an assessment of the ability of its internal fabric to absorb and store thermal energy. Through absorption of heat, the temperature of a material increases – the amount of heat that must be absorbed to raise the temperature of a material by one degree varies by material. This ability to absorb heat is measured by the Specific Heat Capacity of the material (SHC). The SHC is a measure of the amount of heat energy required to raise 1KG of the material by 1 degree Celsius. Those materials that require most heat to increase in temperature have a high thermal capacity (also know as high volumetric heat capacity), such materials are typically dense in composition and ideal for storing thermal energy – these are materials such as brick, concrete and stone.

As a major element of our heating and cooling strategy, through passive solar design, we will seek to employ thermal mass in harnessing the energy of the sun during the day and releasing it during the night when it is needed.

In the Summer, the thermal mass will provide a buffer to over-heating, absorbing thermal energy and reducing the peak temperature whilst moving the time of the peak later into the evening.

In the Winter, high thermal mass buildings will take longer to initially heat up, but with regular occupation will retain the heat for longer and will reradiate heat comfortably overnight when it is required.

Whilst a little late in the day for our barn (it was built 100+ years ago) the orientation of a building to the sun and the suns relationship with the buildings windows, doors and other openings are the focus of passive solar design.

Through effective passive solar design we seek to harness the power of the sun to our advantage whilst protecting inhabitants from its excesses.

So what do we need to consider in our design:

• The strength of the sun at different times of the year. This will be determined by the latitude, altitude and azimuth of the site, with these factors being tempered by shading of the building and weather conditions.
• Ways in which we can maximise the use of the sun in providing heating for the building whilst avoiding over exposure of the interior and inhabitants to solar radiation.
• What technology and techniques will we need to control and manage heat gain, storage and release and then provide ventilation and system reset.

The basic structure of a passive solar heating system is:

Solar radiation enters the the building via glazed windows, it is absorbed by the thermal mass of the building – its masonry walls and floors. Once stored within the thermal mass of the structure technology is employed to ensure that the heat is retained (through good insulation of the envelop of the building) and utilised in an effective and targeted manner (through well designed radiation, convection and conduction paths and optionally energy consuming methods such as fans and air blowers).

A direct gain system is one where the main means of thermal gain is through direct heating of the thermal mass by solar radiation entering the building through windows and being absorbed by the thermal mass. An indirect gain system is one where solar radiation is captured and stored in a component of the building that has a high thermal mass and from there released to areas within the building that require heating.