How to use solar energy for air-conditioning, save billions and cut emissions

Question: if you can use solar power for cooling and air conditioning then why doesn't everybody do it? After all it's a match made in heaven: just when it's so hot you need the aircon, there's lots of solar energy around.

Answer: it's fairly new, not well known and there is a relatively high upfront cost. This makes it a candidate for some sort of net metering or feed in tariffs to kickstart the market, if ever I saw one.

Yesterday I wrote about how architects can use passive solar techniques to design zero carbon buildings and/or drastically cut the need for air-conditioning in warm/hot climates.

In this article I'm going to run you through the technology principles and alternatives for active solar cooling, but first let's look at the problem.

The problem: we want to be cool

It's hot. You turn on the aircon or the fan. Your energy bill goes up

According to the NREL, "air conditioning currently consumes about 15% of the electricity generated in the United States. It is also a major contributor to peak electrical demand on hot summer days, which can lead to escalating power costs, brownouts, and rolling blackouts".

The picture is the same in Europe. For example, according to a national market survey by the Hellenic Ministry of Commerce, about 95% of air-conditioning sales in Greece occur in the period of May-August and reach about 200,000 units (primarily small-size split-type heat pumps) every year. The use of air-conditioning units in summer causes peak electric loads that periodically result in power shortages in large areas of metropolitan cities like Athens.

In southern European countries there is a well-established connection between the growth of peak power electricity demand in summer and the growth of air-conditioning sales in the small and medium-size market.

The picture is the same throughout the world whenever there is hot weather. It leads to ugly city views like this one (right).

The fact that peak cooling demand happens at the same time as high availability of solar energy offers an opportunity to exploit solar thermal technologies that can match suitable solar cooling technologies (i.e., absorption, adsorption, and desiccant cooling), cut emissions from the burning of fossil fuels and in the longer run save billions of dollars in fuel costs.

The technical solutions

Space cooling uses thermally activated cooling systems driven (or partially driven) by solar energy. The two systems are:

1. Closed-cycle:  a heat-driven heat pump that operates in a closed cycle with a working fluid pair, usually an absorbent-refrigerant such as LiBr-water and water-ammonia, or an adsorption cycle using sorption such as silica gel; two or more adsorbers are used to continuously provide chilled water;
2. Open-cycle: solar thermal energy regenerates desiccant substances such as water by drying them, thereby cooling the air. Liquid or solid dessicants are possible. a combination of dehumidification and evaporative cooling of air.

Desiccant cooling system assisted by solar energy from air collectors and PV moduls. Pompeu Fabra Library (Mataró, Spain) | Source: AIGUASOL

More case studies below the techy section, next.

Absorption NH₃/H₂O

The single stage, continuous absorption refrigeration process works as follows: The working fluid (WF), mainly ammonia and water, is boiled in the generator, which receives heat from the solar collectors at 65–80°C. Mainly ammonia, but some water leaves and is condensed at the water cooled condenser (25–35°C). The boiling working fluid in the generator has therefore to be exchanged continuously using the pump to deliver strong working fluid with a concentration of 40% ammonia, from the absorber via the working fluid heat exchanger, which heats it to 50–65°C taken from the weaker fluid leaving the generator.

The latter, now cooler, is led to the absorber, and leaves the absorber at c.35°C. Meanwhile, the condensed refrigerant ammonia has left the condenser and is injected into the evaporator by the refrigerant control valve. This works at low pressure level (2–4 bar), and the refrigerant boils and evaporates. The cold vapour flows into the absorber which absorbs it, combines it with the working fluid, and sends in back to the generator.

The thermal coefficient of performance (COP_thermal) describes the relation between the profit (cooling capacity) and the expense (heat from the collectors): COP_thermal = Q_cooling / Q_heating

Absorption H2O/LiBr

This system employs a refrigerant expanding from a condenser to an evaporator through a throttle in an absorber/desorber combination that is akin to a “thermal compressor” in a conventional vapour compression cycle. Cooling is produced through the evaporation of the refrigerant (water) at low temperature. The absorbent then absorbs the refrigerant vapour at low pressure and desorbs into the condenser at high pressure when (solar) heat is supplied. In this a single-effect absorption system liquid refrigerant leaving the condenser expands through the throttle valve into evaporator taking its heat of evaporation from the stream of chilled water and cooling it.

The vapour leaving is absorbed by an absorbent solution entering dilute in refrigerant (strong absorption capability) leaving rich in refrigerant (weak absorption capability), where it is pumped via a heat exchanger to a desorber which regenerates the solution to a strong state by applying heat from the solar-heated water stream, causing the desorption of refrigerant. It condenses in the condenser to liquid, then expands into the evaporator. The absorber and condenser are cooled by streams of cooling water to reject the heats of absorption and condensation respectively.

Adsorption

Adsorption substances are working pairs, usually water/silica gel. The solid sorbent (gel) is alternately cooled and heated to be able to adsorb and desorb the refrigerant (water). A sequence of adsorbers in deployed to use the heat from one to power another. The cycle is (refer to the schematic diagram, right): refrigerant previously adsorbed in one adsorber is driven off through the use of hot water (may be solar-heated) (right compartment).

It then condenses in the condenser and the heat of condensation is removed by cooling water. The condensate is sprayed in the evaporator and evaporates under low partial pressure, producing cooling power. The refrigerant vapour is adsorbed into the other adsorber (left) where heat is removed by cooling water.

Open cycle liquid desiccant cooling

Humidity is removed from the process air by the desiccant, which is then regenerated by heat from an available source, e.g. solar. Both solid and liquid hygroscopic materials may be used in the dehumidification of conditioned air.

Liquid desiccant systems can store cooling capacity by means of regenerated desiccant. Solar thermal energy is used whenever available to run the desorber and its associated components (hot water-to-solution heat exchanger, air-to-air recuperator, pump) to concentrate hygroscopic salt.

Later, when needed, this is used to dehumidify process air. This method of cold storage is the most compact, requires no insulation and can be applied for indefinitely long time periods.

Solid desiccant air handling unit 

Here, two air channels are mounted on top of each other. The outdoor air enters (A) where the sorption wheel with a silica gel surface dehumidifies is (B) and transfers heat from the outgoing air (C), rehumidifies it to the correct level then enters the conditioned space, increases its enthalpy by internal heat sources and moisture, and leaves as return air (G) where moisture (H and J) and heat (I) are removed as necessary and it is expelled (L).

Highly effective solar collectors should be used for the heat regeneration. The Middle European climate allows an enhancement of the adiabatic cooling mode.

Relation between the cooling capacity and the regeneration heat: COP_{thermal, plant} = Q_c,plant / Q_heat

Relation between the cooling load and the regeneration heat: COP_{thermal, build} = Q_{c, build} / Q_heat

Desiccant-Enhanced Evaporative (DEVAP) Air Conditioner

NREL, AILR Research, Inc. and Synapse Product Development have developed the DEVAP air conditioner. This consists of two stages: dehumidifier and indirect evaporative cooling.

Water is added to the tops of both; liquid desiccant is pumped through the first. Some outdoor air is mixed with return air from the building to form the supply air stream, which flows left to right through the two stages. In the dehumidifier, a membrane contains the desiccant while humidity from the supply air passes through it to the desiccant, which is also in thermal contact with a flocked, wetted surface that is cooled as outdoor air passes by it, causing the water to evaporate and indirectly cooling the desiccant.

In stage 2, the supply air passes by a water-impermeable surface that is wetted and flocked on its opposite side, providing indirect evaporative cooling. A small fraction of the cool, dry supply air is then redirected through the second-stage evaporative passages to evaporate water from the flocked surface and is then exhausted.

Evaluation

More information and a simplified evaluation tool called "Easy Solar Cooling" can help assess the cost performance of different technologies and system designs under different operating conditions. See: http://www.solair-project.eu/218.0.html

Two examples of solar cooling in practice

Video of a large scale solar cooling in South Africa:



Solar cooling in Italy

Solar cooling for a department store in Rome: The 3,000 m^{2</"600"sup> of collector area run a 700 kW chiller. Photo: Metro Cash & Carry} 

The Italian minister for economic development, Claudio Scajola, inaugurated this innovative, energy saving project in Rome on a Metro Cash & Carry building. The installation uses solar energy to cool down the wholesale outlet during summer and to heat it in winter. With its 3,000 m² of solar collectors – provided by the Italian Riello Group – this system on the store's roof is one of the biggest in Italy. It reduced the store's energy consumption by 12%, Dominique Minnaert, managing director of METRO Cash & Carry Italy, was quoted saying in the press release.

The system was designed by the British company AP Engineering Services. The chiller, with a power of 700 kW, was provided by the US-American Carrier Corporation, a leader in the areas of heating, ventilation, air conditioning and refrigeration systems. The cooling tower came from Evapco Europe, a specialist for industrial and commercial cooling equipment with headquarters in Belgium and Italy. The installation in Rome is part of Metro´s project “Energy Saving Today”. Its goal is to optimize performance of storage technology and therefore reduce energy consumption.

I fervently hope that many companies and organisations, not to mention individuals take up this exciting range of technologies.

David Thorpe is the author of 

• Solar Technology: The Earthscan Expert Guide to Using Solar Energy for Heating, Cooling and Electricity 
• Energy Management in Buildings: The Earthscan Expert Guide
• The 'One Planet' Life: A Blueprint for Low Impact Development
• Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency, and
• Energy Management in Industry: The Earthscan Expert Guide

How to save millions on air conditioning by designing passively cooled buildings

Air conditioning is by far the greatest consumer of electricity in buildings in hot countries, but it needn't be so.

Architects designing buildings for regions that otherwise would require air conditioning can use passive solar techniques to keep them cool, and in many cases successfully eliminate the need for expensive air conditioning.

Right: A design for a mosque using passive solar and evaporative cooling.

Passive solar cooling operates in two stages:

1. Do your best to prevent the sun from reaching the building or gaining the interior of the building during the periods when it is in danger of overheating.
2. Then employ passive techniques to remove unwanted hot air.

Different techniques are available depending upon the climate, i.e. whether it is dry or humid.

Passive cooling for warm/hot, dry climates

Here is a summary of the techniques available for hot, dry climates with a large temperature variation from day to night use:

• high interior thermal mass;
• exterior superinsulation;
• highly-reflective OR green roofs, with insulation and a radiant barrier beneath;
• night ventilation;
• phase change materials;
• air vents;
• diode roofs;
• roof ponds;
• wind towers;
• ensure correct exterior shading on windows;
• closing windows and deploying shutters at sunrise to keep out the hot daytime air;
• direct evaporative cooling.

Above: How a wind tower on a building is used for cooling cooling.



Cooling is provided by radiant exchange with the massive walls and floor plus optional techniques explored in more detail below. Open staircases, etc. may provide stack effect ventilation, but observe all fire and smoke precautions for enclosed stairways.

Curved roofs and air vents are used in combination where dusty winds make wind towers impracticable: a hole in the apex of a domed or cylindrical roof with a protective cap over the vent directs the wind across it and provides an escape path for hot air collected at the top.

Arrangements may be made to draw air from the coolest part of the structure as replacement, to set up a continuous circulation and cool the spaces.

Passive cooling for warm/hot and humid climates

This is a summary of the techniques available for warm and humid climates, with little temperature variation from day to night:

• airtight and superinsulated construction;
• a radiant barrier beneath the roof deck;
• highly-reflective or green roofs, with insulation beneath;
• phase change materials;
• daytime cross-ventilation to maintain indoor temperatures close to outdoor temperatures.
• fresh air brought in through a dehumidifier through the crawlspace or basement by using underground pipes or use solar-powered absorption chillers;
• avoid drawing in unconditioned replacement air that is hotter or more humid than interior air;
• avoid open areas of water such as pools;
• the Passivhaus standard permits 15 kWh/m².yr of cooling energy to be used, which has proven to be sufficient in almost all cases because the Passivhaus system is highly effective in reducing unwanted heat gains;
• As above, curved roofs and air vents are used in combination where dusty winds make wind towers impracticable.

Large buildings will require detailed modelling. Power may be required for anti-stratification fans and ducts.

Shading tactics

Some tactics for providing shading to prevent the sun from reaching the building are:

• Covering of rooves and courtyards with deciduous vegetation (allowing winter access) such as creepers or grapevines permits evaporation from the leaf surfaces to reduce the temperature. At night, this temperature is even lower than the sky temperature.
• Green rooves, earthenware pots laid out on the roof, and highly-reflective surfaces (e.g. painted with titanium oxide white paint/whitewash) are all techniques practiced widely.
• Coverings that in the daytime insulate the roof but automatically withdraw at night exposing the roof to the night sky, allowing heat to leave by radiation and convection.
• Horizontal overhangs or vertical fins prevent overheating while preserving natural daylighting.
• For east and west walls and windows in summer: vertical shading and/or deciduous trees and shrubs.
• For south-facing windows: horizontal shading.
• Shutters, closed in the day.
• Highly textured walls leave a portion of their surface in shade.

Right: A summary of different shading types.

Natural ventilation

The design of natural ventilation systems varies on building type and local climate. The amount of ventilation depends on the careful design of internal zones, and the size and placing of openings.

Wind-induced ventilation is helped by siting the ridge of a building perpendicular to summer wind direction, with minimal obstruction to the wind.

In a multi-storey building, the rooms or zones on the outside faces may be separately controlled, depending upon the degree of sophistication present in the building, its size and location. If hot air is present above a set temperature, it is allowed to escape at whatever rate is necessary to preserve the comfort of this zone’s occupants, via controlled venting into a vertical space – atrium or stairwells/lift shafts – positioned centrally or on corners (with glass sides to aid the process).

At the top of this space, louvres, perhaps in clerestories or skylights, allow a controlled amount of hot air to escape, again, at whatever rate is necessary for comfort of the whole building’s occupants. Basement windows allow cool air in.

• Offset inlet and outlet windows across the room or building from each other.
• Make window openings operable by the occupants but controlled by the building management system.
• Provide ridge vents at the highest point in the roof that offers a good outlet for both buoyancy and wind-induced ventilation.
• Allow for adequate internal airflow.
• In buildings with attics, ventilate the attic space to reduce heat transfer to conditioned rooms below

It's not always possible for a building to be completely natural ventilated. In such cases fan assistance is required. Thermostats, dampers and fans would be connected to a building energy management system.

Ventilation chimneys include caps to prevent backdrafts caused by wind. These adjust according to the wind intensity and direction and increase the Venturi effect.

Turbines may be deployed to increase ventilation. Self-regulating turbine models are available. There are several styles of passive roof vents: e.g., open stack, turbine, gable, and ridge vents, which utilise wind blowing over the roof to create a Venturi effect that intensifies natural ventilation.

Bernoulli's principle

This uses wind speed differences to move air, based on the idea that the faster air moves, the lower its pressure. Outdoor air farther from the ground is less obstructed, with a higher speed, and thus lower pressure. This can help suck fresh air through the building.

Bernoulli’s principle multiplies the effectiveness of wind ventilation and is an improvement upon simple stack ventilation. However, it needs wind, whereas stack ventilation does not. In many cases, designing for one effectively designs for both.



The BedZED development in south London (above) utilises specially-designed wind cowls which have both intakes and (larger) outlets; fast rooftop winds get scooped into the buildings. The larger outlets create lower pressures to naturally suck air out.

Solar chimneys for cooling

Solar chimneys are employed where the wind cannot be relied upon to power a wind tower. The chimney's outer surface (painted black and glazed) acts as a solar collector, to heat the air within it. (It must therefore be isolated by a layer of insulation from occupied spaces.)

Pre-cooling air with ground source intakes

Right: Earth-air tunnel.

Also known as an earth-air tunnel this is a traditional feature of Islamic and Persian architecture.

It utilises pipes buried a few meters down, or underground tunnels, to cool (in summer) and to heat (in winter) the air passing through them.

They can lower or raise the outside replacement air temperature for rooms which are buffer zones between the interior and exterior temperatures.

Trombe wall effect for cooling and heating

A double skin façade is employed, the outer skin of which can be glass or PV panels. The cavity between the array and the wall possesses openings to indoors and outdoors at both high and low levels. operate more efficiently anyway).

Evaporative cooling

Evaporative cooling is used in times of low or medium humidity. As water is evaporated (undergoing a phase change to water vapour), heat is absorbed from the air, reducing its temperature. When it condenses (another phase change), energy is released, warming the air. This is the same for all phase change materials.

Right: passive solar cooling with a courtyard.

Evaporative cooling can be direct or indirect; passive or hybrid.

• Direct: the humidity of the cooled air increases because air is in contact with the evaporated water – can be applied only in places where relative humidity is very low.
• Indirect: evaporation occurs inside a heat exchanger and the humidity of the cooled air remains unchanged – used where humidity is already high.
• Passive: where evaporation occurs naturally; incoming air is allowed to pass over surfaces of still or flowing water, such as basins or fountains;
• Hybrid: where mechanical means are deployed to control evaporation.

Phase change materials (PCMs)

PCMs utilise the air temperature difference between night and day. In the daytime, incoming external air is cooled by the PCM-storage module, which absorbs and stores its heat by changing its phase state (e.g. solid to liquid).

At night-time the substance reverts to solid form, releasing its heat by being cooled by the now cooler external air. Commercially available PCMs are chosen based on the temperature of their phase change relative to that required in the space to be moderated.

Night cooling

Night ventilation, or night flushing relies upon keeping windows and other openings closed during the day but open at night to flush warm air out of the building and cool thermal mass which has heated up during the day.

It relies upon significant temperature differences between day and night time (which must be below 22°C / 71°F) and some wind movement.

The above combines edited extracts from one of my published books, Solar Technology, and a forthcoming title: Passive Solar Architecture Reference Pocketbook.

David Thorpe is the author of 

• Solar Technology: The Earthscan Expert Guide to Using Solar Energy for Heating, Cooling and Electricity 
• Energy Management in Buildings: The Earthscan Expert Guide
• The 'One Planet' Life: A Blueprint for Low Impact Development
• Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency, and
• Energy Management in Industry: The Earthscan Expert Guide

4 Ways to Plan Neighborhoods and Buildings to Minimize Energy Use

Conventional buildings consume much energy for heating and cooling to protect them from the temperature effects of climate and seasons. But some basic thought and planning, in combination with these 10 passive solar building design techniques, can help to radically reduce these energy costs. Here's 4 key ideas:

1. Optimize the spatial layout

 

Inappropriate (left) and appropriate (right) spatial layouts for settlements in hot climates. Grid layouts borrowed from other climates, and wide spacing of buildings, do not provide shade or wind shelter. Organic, non-grid layouts do provide shade and can be designed to block winds, preventing issues with wind funnelling. Credit: author.



Right: Sample layout for housing estate in higher latitudes such that each property has both privacy and an equator-facing aspect and roof to maximise potential use of solar energy. Grey circles are trees, grey lines are hedges (preferably) or fences. Credit: author.

2. Optimize the building form and layout

A low surface area to volume (S/V) ratio is optimal for a passive, low-carbon building. This is the ratio between the external surface area and the internal volume.

Compactness C = Volume / Surface Area

Size is also a factor: a small building with the same form as a larger one will have a higher S/V ratio. Buildings with the same U-values, air-change rates and orientations but differing S/V ratios and/or sizes may have significantly different heating demands. This has the following consequences:

• small, detached buildings should have a very compact form (square is close to the perfect optimum, the circle);
• larger buildings may have more complex geometries;
• high S/V ratios require more insulation to achieve the same U-/R-value.

In temperate zones, aim for an S/V ratio ≤ 0.7m²/m³.

Form factor

The ratio of the usable floor area, F, to above-grade enclosure area E is more useful, because it favours buildings that require less floor-to-floor height.

Form factor = F/E

The more compact the form, the higher the ratio, which is better. Large buildings (e.g., 172,800 ft² over 12 stories) have a much more efficient form than small buildings or large high-bay buildings for heating load (but not cooling, where the opposite is true).

This metric permits comparisons of the efficiency of the building form relative to the useful floor area. Achieving a heat loss form factor of ≤3 is a useful benchmark guide when designing small Passivhaus buildings. This also reduces the resources required and the cost. Most building uses do not require volume but floor area. This metric also does not include the ground contact area, but does include the roof.

A building with a more complex form is also likely to have a higher proportion of thermal bridges and increased shading factors that will have an additional impact on the annual energy balance.

The effect of form on total energy consumption for a given floor area is reduced as buildings increase in size. Besides permitting greater design flexibility, this lets designers use daylighting and natural ventilation cooling strategies also to reduce energ demand, as these require one dimension of the building to be relatively narrow (between 45 and 60ft (14–18m).

Example:

For a small office of 20,000 ft2 (1800 m2) a narrow two-storey form, ideal for natural ventilation and daylighting, may have a form factor ratio of 0.88, whereas a deep square plan have one of 1.02. For the former to have the same enclosure heat loss coefficient as the latter, its overall average enclosure R-value would need to be 1.02/0.88 = 16% higher. This would require a significant increase in the opaque wall area R-value, a reduction in window area, or a more expensive window.

 

An increase in the S/V ratio of 10% (the building in the middle) would require 20mm of insulation more than the good form on the left to achieve the same level of insulation. The one on the right (a 20% higher S/V ratio) would require an extra 40mm of insulation.



Optimum room layouts in dwellings according to the climate.

3. Adapt the dwelling forms and room layouts according to latitude

For latitudes above 25°: the sun-facing glazing area should be at least 50% greater than the sum of the glazing area on the east- and west-facing walls. Orientation is long on the east-west axis, which should be within 15 degrees of due east-west. At least 90% of the sun-facing glazing should be completely shaded (by awnings, overhangs, plantings) at solar noon on the summer solstice and unshaded at noon on the winter solstice. The room plan should – if it is a dwelling – incorporate the main living rooms on the equator-facing side, with utility rooms, less used rooms and garage if any on the north side. Morning rooms are typically bedrooms. On the side away from the equator windows should be kept to a minimum and as small as possible for lighting to minimise heat loss. This wall should also have high thermal mass or/and be externally insulated, to retain heat in the building.

For latitudes less than 25° or where topography significantly impacts insolation, the opposite should be the case. Bedrooms, for example, need light in the morning. The whole building needs to be protected from low angle heat.

Around 25° there is some leeway depending on local conditions. In these mid-latitudes different parts of a building may be used in the winter and summer, as equator-facing rooms become too hot and occupancy is switched in summer to rooms on the non-equator-facing side (not shown in the above left plan).

Table: The shape of the building has different requirements according to the local climate:

Climate	Elements and requirements	Purpose
Warm, humid	Minimise building depth	for ventilation
	Minimise west-facing wall	to reduce heat gain
	Maximise south and north walls	to reduce heat gain
	Maximise surface area	for night cooling
	Maximise window wall	for ventilation
Composite	Control building depth	for thermal capacity
	Minimise west wall	to reduce heat gain
	Limited equator-facing wall	for ventilation and some winter heating
	Medium area of window wall	for controlled ventilation
Hot, dry	Minimise equator-facing and west walls	to reduce heat gain
	Minimise surface area	to reduce heat gain and loss
	Maximise building depth	to increase thermal capacity
	Minimise window wall/window size	to control ventilation, heat gain and light
Mediterranean	minimise west wall	to reduce heat gain in summer
	Moderate area of equator-facing wall	to allow winter heat gain
	Moderate surface area	to control heat gain
	Small to moderate window size	to reduce heat gain but allow winter light
Cool temperate	Minimise surface area	to reduce heat loss
	Moderate area of pole-facing and west walls	to receive heat gain
	Minimise roof area	to reduce heat loss
	Large window wall	for heat gain and light
Equatorial upland	Maximise north and south walls	to reduce heat gain
	Maximise west-facing walls	to reduce heat gain
	Medium building depth	to increase thermal capacity
	Minimise surface area	to reduce heat loss and gain

4. Optimize the roof shape and orientation

In hot climate zones, vaulted roofs and domes dissipate more heat by natural convection than flat roofs. They give greater thermal stability and lower daytime temperature. The best orientation requires that the vault form receive maximum daily solar radiation in winter and minimum in summer.

A north-south axis orientation for a vaulted roof is better for winter heating, receiving the minimum direct solar radiation in the summer, while an east-west axis orientation will maximise summer heating, receiving the most irradiation in the morning and evening. The results are summarised by example for a 30° latitude site below.

Table: The effect of vault orientation on seasonal direct solar radiation.[i] CSR = Cross Section Ratio. This is the ratio between vertical height of the vault and the horizontal width.

Orientation	Season	Loss of direct solar radiation (%)
		CSR1 = 0.5	CSR1 = 0.8	CSR1 = 1	CSR1 = 1.25	CSR1 = 2
W-E	Summer	12.4	20.1	23.9	29	37.8
	Winter	9.8	17	19.6	23.2	30.4
N-S	Summer	17	28.6	35.1	42.1	56.4
	Winter	6.3	7.1	8	8.9	10.7
NE-SW	Summer	14.7	23.9	29.3	34.8	45.6
	Winter	8.9	13.4	16	18.8	24.1
NW-SE	Summer	14.7	23.9	29.3	34.8	45.6
	Winter	8.9	13.4	16	18.8	24.1

The effect of vault orientation on received seasonal direct solar radiation.

See this related post on passive solar building design techniques.

David Thorpe is the author of 

• Solar Technology: The Earthscan Expert Guide to Using Solar Energy for Heating, Cooling and Electricity 
• Energy Management in Buildings: The Earthscan Expert Guide
• The 'One Planet' Life: A Blueprint for Low Impact Development
• Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency, and
• Energy Management in Industry: The Earthscan Expert Guide

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[i] Mashina, GA and Gadi, MB; Calculating direct solar radiation on vaulted roofs using a new computer technique, Nottingham University Conference Proceedings, 2010. Available at: http://www.engineering.nottingham.ac.uk/icccbe/proceedings/pdf/pf196.pdf

10 stages to a passive solar building from design to build

Some aspects of a zero carbon building in the northern hemisphere, temperate zone.

Passive solar architectural principles have come of age. They have given rise to thousands of buildings of all sizes and purposes around the world, in all climate types, to demonstrate how buildings don't need to consume fossil fuel energy to support their occupants. They can even generate more power, or absorb more carbon, than they use. Below is a ten-step guide to how to go about designing and building one.

But  first, the benefits of passive solar architecture:

• Saves energy and running costs from the start;
• Comfort in all seasons and climates;
• Safe investment and resilience into the future;
• Added value every year through decreased operation costs;
• Longer useful life with high quality standard;
• Contributes to climate change protection.

Thanks to experience of the last 30 years, we know what works in different climates in the world and build costs have reduced to the same or just slightly higher than conventional builds. But lifetime costs are considerably cheaper – on average (dependent on purpose/design) being 87.5% savings (1/8 less to run) on heating and cooling.

Sustainable solar building is also known as passive house (Passivhaus in German), although this is also a strict standard.

There are currently 30,000 Passivhaus structures built around the world. The principle is that the architecture is designed to provide comfort for the occupants with minimum need for additional energy. This is achieved using design tools to establish the needs and requirements of all functions in the building and their inter-relationships. Energy savings are maximised by placing spaces in the most advantageous position for daylighting, thermal control, and solar integration.

This process may also reveal opportunities for multiple functions to share space and reduce the footprint of the building.

General principles of a passive solar building

Buildings should be at least zero carbon on balance, when totalling the impacts of materials, construction, use and demolition. Features of this are to:

• minimise the use of fossil fuel energy during the supply chain and process of construction;
• encourage the use of materials which store atmospheric carbon in the fabric of the building;
• encourage the generation and even export of renewable energy by the building;
• construct and manage it in such a way that it minimises the emission of greenhouse gases during its lifetime and eventual demolition.

Right: possible building designs for maximising the use of daylighting.

Such a building could, over its lifetime, become zero carbon, or even negative carbon by generating enough power to more than make up for the fossil fuels it has used.  To achieve this, the following features are needed:

• favouring the use of ‘natural’ and cellulose-based materials (timber products, and other products made from plant-originating materials);
• making the structure very airtight;
• making the structure breathable;
• making it durable, resilient, low-maintenance, fire- and weather-resistant;
• incorporating a large amount of insulation;
• taking advantage of free, renewable energy. 

10 steps to a zero energy building

So here, now is a summary of 10 steps in the design and build strategy of a zero-energy passive solar building, averaged for any climate zone in the world.

1. Site selection

• Secure optimum location, as free as possible from non-useful shading relative to the seasons and time of day. 
• Research the available solar resource and wind factors for the site using local and freely available data.
• Orient optimally.

Example of a sun chart to view the azimuth angle through a year at a building location.

2. Concept development

• Minimize shade in winter, minimizing parapets, projections, non-transparent balcony enclosures, divider walls etc.
• Choose a compact building structure with low skin to volume ratio. Use opportunities to combine buildings. 
• Use a simple shell form, without unnecessary recesses. 
• Survey and model the expected internal and external heat gains and cooling requirements, and other building energy loads.
• The following energy performance targets and air changes per hour define the Passivhaus standard and must be met in order for certification to be achieved:
• Specific heating demand ≤ 15 kWh/m²/yr
• Specific cooling demand ≤ 15 kWh/m²/yr
• Specific heating load ≤ 10 W/m²
• Specific primary energy demand ≤ 120 kWh/m²/yr
• Air changes per hour ≤ 0.6 @ n50.
• Optimize glazing, shading and aspect/form according to latitude and climate zone, to maximize the use of daylighting, balancing against the appropriate heat gains.
• Concentrate the utility installation zones, e.g. bathrooms, above or adjacent to the kitchen, in coolest areas in summer (temperate zones) or hottest (hot zones).
• Model and decide on the ventilation scheme making best use of the stack effect and Bernoulli principle. Is additional mechanical ventilation (with heat recovery) needed?
• Thermally separate basement from ground floor (including cellar staircase), make airtight and thermal bridge free. 
• Derive an initial energy use estimate.
• Evaluate the potential for renewable energy technologies: solar thermal, PV, wind, heat pumps, etc.
• Consider use of underfloor heating to save energy (water or electric).
• Check the possibility of government subsidies.
• Commence consultations with the building authority.
• Contract agreement with architects, including a precise description of services to be rendered.

3. Construction plan and building permit planning

• Select the building style – thermally massive or light. Sketch out a design concept, floor plan, energy concept for ventilation, cooling, heating and hot water. 
• Floor plan: short pipe runs for hot/cold water and sewage.
• Consider the space required for utilities (cooling/heating, ventilation etc.).
• Short ventilation ducts: cold air ducts outside, warm ducts inside the insulated building envelope.
• Further calculate and minimize the energy demand, e.g. with the Passive House Planning Package (PHPP) available from the Passivhaus Institut, Darmstadt. Climate data sets are available for most areas in the world which plug into this.
• Plan the insulating thickness of the building envelope and avoid thermal bridges.
• Calculate cost estimate.
• Negotiate the building project (pre-construction meetings).

4. Final planning of the building structure (detailed design drawings)

• Insulation of the building envelope: the absolute U-values will vary according to context (location, form etc), but in general aim for: 
• walls, floors and roofs ≤ 0.15 W/m²K; 
• complete window installation ≤ 0.85 W/m²K.
• Design thermal bridge free and airtight connection details.
• Specify windows that comply with passive house standard: optimize type of glazing, thermally insulated frames, glass area, coating, shading.
• 5. Final planning of ventilation (detailed system drawings)
• General rule: hire a specialist.
• Ventilation ducts: short and sound-absorbing. Air flow velocities below 3 m/s.
• Include measuring and adjusting devices.
• Take sound insulation and fire protection measures into account.
• Air pathways: avoid air current short-circuiting.
• Consider the air throws of the air vents.
• Provide for overflow openings.
• If MHVR/cooling is used, install in the temperature-controlled area of the building shell.
• Additional insulation of central and back-up unit may be necessary. Soundproof the devices. Thermal energy recovery rate should be > 80 %.
• Airtight construction to be checked at every stage.
• The ventilation system should be user-adjustable.
• Optional: ground or water-source heat pump (air or water as medium) and/or air pre-cooling/heating pipes; may be reversible for summer cooling and winter heating.

6. Final planning of the remaining utilities (detailed plumbing and electrical drawings)

• Plumbing: Install short and well-insulated pipes for hot water in the building envelope. For cold water install short pipes insulated against condensation water. Use no greater bore than needed to conserve water and heat.
• Use water-saving fittings.
• Sub-roof vents for line breathing (vent pipes).
• Plumbing and electrical installations: avoid penetration of the airtight building envelope – if not feasible, install adequate insulation.
• Use the most energy-saving appliances/equipment.
• Situate switches for ring mains alongside light switches to enable easy switching off of phantom loads when leaving rooms/building.
• Plan installation of (perhaps wireless) building energy monitoring system.

7. Call for tenders and awarding of contracts

• Plan for quality assurance measures in the contracts.
• Set up a construction schedule.

8. Assurance of quality by the construction supervision

• Thermal bridge free construction: schedule on-site quality control inspections. Take photographs.
• Check of airtightness: all pipes and ducts must be properly sealed, plastered or taped. Electrical cables penetrating the building envelope must be sealed also between cable and conduit. Flush mounting of sockets in plaster and mortar. Take photographs.
• Check of thermal insulation for ventilation ducts and hot water pipes.
• Seal window connections with long-lasting adhesive tapes or plaster rail. Apply interior plaster from the rough floor up to the rough ceiling.
• n50 airtightness test: Have a blower door test done during the construction, when the airtight envelope is complete but still accessible, i.e., before finishing the interior work, but after completion of the electricians' work (in concert with the other trades), incl. detection of all leaks.
• Ventilation system: ensure easy accessibility for filter changes. Adjust the air flows in normal operation mode by measuring and balancing the supply and exhaust air volumes. Balance the supply and exhaust air distribution. Measure the system's electrical power consumption.
• Quality control check of all cooling, heating, plumbing and electrical systems.

9. Final inspection and auditing.

10. Conduct post-occupancy monitoring

...to determine if building performs as expected.

David Thorpe is the author of 

• Solar Technology: The Earthscan Expert Guide to Using Solar Energy for Heating, Cooling and Electricity 
• Energy Management in Buildings: The Earthscan Expert Guide
• The 'One Planet' Life: A Blueprint for Low Impact Development

He is currently working on a Passive Solar Architecture Pocket Reference Book. Acknowledgements to Isover, St Gobain.

We need a definition of Nearly Zero Energy Buildings

A selection of low energy building types in Europe.

Most of the buildings around now will still be here in 2050, so the real challenge is not only making new buildings energy efficient but eco-renovating the old ones.

A new analysis of different pathways to achieving this goal has identified the most cost-effective ones, depending upon the existing housing stock. Although the study is focus on the different member states of the European Union, it could be applicable elsewhere that buildings and climate are similar, since the range of both within Europe is large.

The message is that while a strengthening of regulatory measures is essential, what is really crucial is a much stronger focus on compliance with regulations.

In most cases nobody ever bothers to check whether building regulations have been complied with, let alone conducts post-occupancy evaluations to see whether the expected performance is achieved.

The study uncovers a huge lack of data regarding renovation activities and the energy performance of buildings and calls for a building data observatory, in particular for monitoring the impact of policies.

It finds that the European Energy Performance of Buildings Directive (EPDB) has not performed as well as expected, even in the recast version.

The EPDB requires that from 2019 onwards all new buildings occupied and owned by public authorities in Europe are nearly Zero Energy Buildings (nZEBs) and that by the end of 2020 all new buildings must be nZEBs.

But because there is a wide variety in building culture (including ownership) and climate throughout Europe the Directive doesn't prescribe a uniform approach. Instead it asks member states to draw up specific national roadmaps that reflect their national, regional or local conditions.

Chart of European building stock by country and age:



It's necessary to deeply renovate the existing building stock to highly ambitious levels, in line with long-term energy policy and climate mitigation targets.

But the problem is that the Directive does not contain a clear definition of nZEB renovation.

This study, Laying down the pathways to nearly Zero-Energy Buildings, A toolkit for policy makers, undertaken by ENTRANZE (Policies to Enforce the Transition to nearly Zero-Energy Buildings in the EU-28), attempts to find policies to fill this gap.

According to the ENTRANZE model results for EU-28, the current policy framework could lead to savings of about 20%-23% of final energy demand from 2008-2030. In contrast, more ambitious policies, as developed during this project in extensive discussions with policy makers, would lead to savings of 29-31% in final energy demand.

However, this still does not represent the maximum of achievable effort and policy innovation. Almost half of the final energy demand for heating and hot water is covered by natural gas in 2008.

The research shows that an ambitious policy package could reduce natural gas demand in 2030 by almost 36-45%, potentially halving energy dependency on gas in the building stock by 2030.

The study says that the EPBD needs to make clear that cost-effectiveness must stand as the absolute minimum requirement for existing relations in building codes, and that current activities to improve high quality renovation, that would result in substantial savings, have to be significantly increased to have a lasting impact.

"While nZEB's energy performance level should be cost effective they still have to be more ambitious than cost optimal energy performance levels," the report says.

Chart of cost-optimal building eco-refurbishment in Europe:



It argues that European member states must be challenged to close the gap between the requirements of nZEB targets and the cost of the less stringent levels of current building codes. It should then gradually increase the requirements of the targets for existing buildings and for this a clear definition of "deep renovation" or nZEB is required.

There is also some confusion with standardisation and terminology.

Single family houses show the most potential for the use of renewable energy technologies is more effective in Mediterranean climates (characterised by higher solar radiation). A similar trend applies to office buildings, but with fewer differences between the South and North of Europe because of the higher electricity consumption for auxiliary systems and mechanical ventilation.

Net primary energy saving percentages for cost-optimal and nZEB targets are closer together in residential buildings than in office and school buildings. Multi-family dwellings show lower energy saving potential compared to single houses, due to geometric limits, such as the lower available roof area for solar systems.

Three tools

Part of the problem that the study attempts to tackle is the high initial cost of a deep renovation compared to the energy saving over 30 years. The study uses three tools to analyse this:

• The Data Tool: an in-depth description of the characteristics of buildings and related energy systems in the EU-28 and Serbia.
• The Cost Tool: ia powerful, flexible instrument to analyse the impact of a large number of renovation packages for specific building types in terms of costs and primary energy demand.
• The Online Scenario Tool: the results of alternative scenarios for the development of the building stock and its energy demand in the EU-28 (+ Serbia) up to 2030.

It concludes that while measures required to achieve nZEB-levels may not be cost-effective today, by 2020 they could be. this is especially true of using renewable energy systems for heating and cooling.

Perhaps its most valuable contribution is a country-by-country analysis and set of recommendations. For instance for France it says that:

"Despite five updates of building codes since 1974 for new construction and the fact that the last building code implemented (RT2012) is one of the most stringent in EU29, the specific energy consumption per m² and per heating degree days in buildings in France is still significantly higher than in other EU countries."

The study builds on an earlier one on the definition of nZEBs, concluded in 2011, which just goes to show how regrettably slow movement is on this topic.

This study found that:

"typically, low-energy buildings will encompass a high level of insulation, very energy efficient windows, a high level of air tightness and natural/ mechanical ventilation with very efficient heat recovery to reduceheating/cooling needs.

"Passive solar building design may boost their energy performance to very high levels by enabling the building to collect solar heat in winter and reject solar heat in summer and/or by integrating active solar technologies (such as solar collectors for domestic hot water and space heatingor PV-panels for electricity generation).

"In addition, other energy/resource saving measures may also be utilized, e.g. on-site wind turbines to produce electricity or rainwater collecting systems."

Yet, it found that in 2011, more than half of the Member States did not have an officially recognised definition for low or zero energy buildings.

Four years later the situation is not much better.

David Thorpe is the author of 

• Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency, and
• Energy Management in Buildings: The Earthscan Expert Guide

Bristol may become the UK's second One Planet City

Next year Bristol is to be Europe's Green Capital City (this year it is Copenhagen). A partnership of 600+ local organisations is considering using the energy generated by this to launch a proposal that Bristol adopts the target of becoming a One Planet City.

On November 24 I attended a gathering of representatives to discuss this exciting prospect, which culminated in a decision to take the proposal to the next level.

The idea for One Planet Bristol originated with the Green Capital Partnership, which has been in existence as a network for four or so years.

Chris Richards, who first came up with the idea, put it like this: "If you imagine a circle for the earth, and Bristol's 550,000 inhabitants as taking up say a fraction of a degree of their share of the earth's resources, getting their consumption down to a proportionate level would be no mean achievement.

"It would inspire others, and if it inspired Europe to do so, that is maybe 20% of that circle. The rest of the world could follow later." By drawing a simple cartoon of this concept he illustrated it perfectly and succinctly.

Herbert Girardet, co-founder of the World Future Council and author of Creating Regenerative Cities, who lives just outside the city, spoke first, giving the examples of Adelaide and Copenhagen to draw upon. "Whereas it took Copenhagen 30 years to get to where it is now, and it is by no means at the 'one planet' stage, it is taking Adelaide 10 years to achieve its targets," he said.

These targets are based on a consultation exercise Girardet carried out with the city three years ago, yielding a comprehensive set of objectives. "But even when it achieves these," he said, "it will still only have reduced its ecological footprint from four to 2.5 planets. Australians have per capita the highest average ecological footprint of any nation's citizens."

Amongst the objectives are the turning of the city's food waste into 180,000 tonnes of compost per year which is used to feed 20 hectares of local farmland that contributes to the food supply of the city. There are also targets of 1GW of solar installations, a Metro, and targets for culture, cycling and biodiversity.

Bioregional

Herbert Girardet was followed by Sue Riddlestone, from Bioregional, which is this month celebrating its 20th anniversary. Bioregional is a consultancy which helps its clients around the world reduce their ecological footprints using the one planet living schema it has evolved.

"We try to make it easy to do the right thing and hard to do the wrong thing," was how she summarised their design approach. This is based around ten principles, amongst which, if applied to Bristol, would be to decarbonise the energy supply by 2030, move to a closed loop resource use system, provide local food, and promote local culture and happiness.

"It would be necessary to move from a per capita emissions rate of 12 tonnes of carbon now to 1.5 tonnes by 2050," she said. She outlined Bioregional's work in Brighton, the first UK city to announce a strategy to move towards one planet living, which has two plans, one for the city and one for the council.

Darren Hall spoke next, as facilitator for the city's bid to win the European Green Capital Award for Bristol as manager of the Partnership, Darren is also the editor of local magazine Good Bristol, and a Green Party candidate.

His key message was that it is "leadership, leadership, leadership" that is required. Three years ago the Partnership submitted to the council a proposal for greening the city that met with no response, "because there was no engagement with the city council on the ideas," event organiser David Parkes said afterwards.

"This time it must be different," Hall said. "We must connect with big business, with grass roots and with the council. We won the Green City bid because we submitted a plan to accelerate green progress in the city up to 2020. We can use the energy of next year to take it further."

He described the value of the 'one planet' concept as a powerful uniting and communication tool that avoids "the tragedy of the commons" when many people pool ideas, diluting and compromising them in the process. "One planet living can be the one rule that rules them all," he said, paraphrasing Tolkein's Lord of the Rings.

I also briefly outlined the work of the One Planet Council, of which I am a patron, and the experience of Wales, which has an aspiration to be a One Planet nation, and which has adopted a One Planet Development planning condition as a first step, and is now developing a Well-Being of Future Generations Bill as a next step.

Pre-requisites for a one planet city

Delegates at the meeting, of which there numbered around 50, compiled a set of pre-requisites for the process of adopting a one planet city aspiration for the city. These include:

• the need for a detailed roadmap;
• a set of independently verifiable standards or measurements;
• bringing local businesses on board with the promise of jobs and a 'green economy';
• a sustainable transport plan;
• ward by ward meetings to gather ideas and build grassroots support;
• the city council as a facilitator not as a leader;
• affordability and finance;
• knowledge of what it is within the power of the council to do;
• support from national government to help with the rest.

Above all it was felt that the banner of 'one planet living', being easy to understand, can raise the profile of the city, and give it something to aim for after its year in the European green spotlight, which would therefore provide a springboard for greater things.

Bristol's mayor is the independent, and green-minded George Ferguson. Bristol is also part of the Core Cities group in the UK, which coincidentally was also meeting yesterday with the national government's William Hague to demand more devolution of powers, including tax-raising powers. The possession of these additional powers could make the road to one planet city status slightly easier.

Another factor in its favour is that it has its own currency, the Bristol Pound, in which Ferguson takes his entire salary. This helps to boost the local economy, keeping money circulating within it.

On the other hand, Bristol is scheduled to double its population within 20 years, being a popular destination, despite being almost as expensive as London to live in. This would make the task of reducing its footprint much harder.

Perhaps they should make a willingness to adopt a 'one planet' lifestyle a condition of being allowed to move to the city.

The Green Capital Partnership will now consider the next steps to take.