Solar Architecture in Cool Climates

Colin Porteous

• Publisher: Routledge
• Year: 2012
• ISBN:
  9781136556999
• Edition: 1
• Topic: Architecture
• Subtopic: Architecture General

'A must-read for practitioners, teachers and others interested in or working with energy use in the built environment, including a delightful set of examples'
Ann Grete Hestnes, former President of the International Solar Energy Society

Solar Architecture in Cool Climates is an invaluable primer on low energy building design, combining accessible information with convincing arguments enabling new techniques to be implemented in daily practice.

Approaching the topic in a thematic manner, the book provides inspiration, an understanding of key principles and technical data on the design of solar buildings in higher latitudes. The text is enlivened through direct experience of case studies from Europe and North America dealing with new-build, retrofitting and conceptual projects that outline future potential (the principles being equally applicable to equivalent southern latitude locations.

The authors examine the dilution of additional costs through different strategies, the tensions between energy efficiency and environmental quality, and the proactive control of energy in building design. Promoting flexibility and opportunity to a diverse audience, including those who use, procure and finance buildings, the book aims to bring the design of 'green' solar buildings in cool climates from special interest status into the mainstream. Broader environmental issues relating to solar architecture are addressed in the final chapter, again drawing on case studies from the authors' own wide experience.

Solar Architecture in Cool Climates is written for architects and other building designers, students of architecture and other professionals interested in sustainable architecture, renewable energy and engineering.

Chapter 1 North Sun Context

The case for solar architecture in cool climates rests to a large extent on a truth that was revealed to the international solar community well over two decades ago and then followed up by many other solar scientists and architects keen to promote the ‘north sun context’. It is therefore fundamental to the thrust of this book and involves the presentation of certain thermal principles at the outset. Fortunately, these are not excessively daunting.

In 1981 at the International Solar Energy Society (ISES) World Solar Congress in Brighton, UK, a paper was presented with analytical information that indicated that northern European latitudes could be better for solar space heating than southern ones (MacGregor, 1981). This is not actually as surprising as it might seem, but requires explanation. For many solar energy applications in the northern hemisphere, both electrical and thermal, the potential increases with the supply of solar radiation as one moves south – at least to the vicinity of the Tropic of Cancer. For example, this is the case for solar photovoltaic (PV) arrays, provided almost all the collected electrical energy can be utilized. The same applies to solar heated water for hygienic and utilitarian uses, where there is a year-round thermal demand. In warmer and sunnier climates, however, there is more demand for cooling spaces than for heating them. Solar energy can still be used to offset cooling by other means, but its usefulness for heating becomes marginal. This generalization is of course subject to topography and regional climatic factors. A town located at a high altitude, but relatively low latitude and in a continental setting that is subject to high daily and seasonal swings in temperature, may have both heating and cooling demands. In essence, the north is better trend for solar displacement of space heating acknowledges that as latitude rises, the solar supply diminishes. But it does so more slowly than the increasing intensity of climatic factors that drive the demand – lower ambient temperature and higher wind speeds, especially accompanied by rain. It has to be accepted that the relative merits of latitude are not immediately obvious. Indeed, in the 1970s a view or prejudice against solar space heating in the north of the British Isles was certainly apparent.

Latitude Myths Challenged

Going back several decades, the UK section of the International Solar Energy Society (UK-ISES) published a review of solar energy (UK-ISES, 1976). It stated: ‘South-west England and south-west Wales are the best areas for solar space heating from the meteorological point of view.’ A government report in the same year (Long, 1976, pp25–44) was worthy in some respects, but naïve and misleading in others. It recognized the supply–demand dilemma, but omitted to mention the significance of demand extending the duration of the heating season, and did not engage with the potential for using unheated glazed spaces as a means of preheating air for ventilation. The term ‘solar ventilation preheat’ is now commonly used. It is possible for solar energy to tackle a thermal niche market, which complements other less sensitive enablers of energy efficiency such as insulation.

The report also rightly emphasized the issue of long-term and medium-term thermal storage, but was incredibly tentative and misleading with respect to solar energy and architecture. For example, having given complete misinformation with respect to the ‘Trombe’ wall (see Chapter 4), the second part of the following statement probably belongs in the territory of wishing in hindsight that it could have been erased from the record (Long, 1976, p37): ‘In practice, a very important distinction is that flat-plate collectors [

1.1(1)] can be installed on suitably oriented existing buildings, while, by definition, techniques involving novel architectural design are applicable only to new properties.’ That is hard to beat for myopic ignorance. Even though solar heating at that time was mainly contemplated from an additive engineering perspective, the logic of denying existing buildings scope for an integrative architectural one is obscure. At any rate, the received wisdom that opportunities for solar heating in the UK were confined to the south still prevailed at the time of MacGregor’s counter proposition at Brighton. A book published that year (Oppenheim, 1981), which set out to explore the potential for solar buildings in cold northern climates, stated: ‘… the most favourable areas lie to the south of Britain, and the least favourable areas to the north.’ Oppenheim supported his assertion with numbers, similar to the ones used in the scientific reports five years previously. Therefore, in mounting a challenge, a limited amount of mathematical investigation is necessary.

Numbers, which are used evidentially, must have a sound basis. However, the methodology used by UK-ISES and Oppenheim relied on a proportional relationship, both parts of which were fundamentally flawed. A crude index of climatic suitability was derived from a supply–demand ratio of ‘global solar radiation’ to ‘degree days’

1.1(2). In the first place, the warmer the climate, the more this index will indicate strong solar potential for heating. Yet, in reality, the reverse is true. At the point when there is minimal demand for heat, there is minimal scope for the solar supply to play a useful part. Most, if not all, of the numerator then has to be discounted. Conversely, the supply should all be counted in as long as there is a heating demand. But a second flaw is that the demand side of the ratio assumes a standard length of heating season for all locations. The same number of days is used everywhere, although the UK-ISES report did acknowledge that northern UK could have a percentage of ‘degree days’ in summer twice that of a sunnier south. So both the supply and demand is less than it should be as one moves north, while not all the supply computed is likely to be useful in the south. For example, the relevant period of time for solar radiation and ‘degree days’ might be six months for London, but at least nine months for the Western Isles in Scotland

1.1(3). Indicative ratios would improve for both locations as a building becomes more energy-efficient, the proportional gap diminishing somewhat and both heating seasons shrinking. It is always subject to all the solar supply being able to usefully contribute to heating, which becomes less likely the warmer the climate and the more efficient the construction.

If the impacts of wind and rain are also evaluated, the contrast will increase. A further problem is that the totals for degree days are based on an initial assumption with regard to the energy efficiency of the buildings. The more thermally efficient a building becomes, the shorter will be the period in each case, as well as the lower the gap in temperature driving the need for heat. But a significant geographical time difference would remain and, in reality, the relativity of supply and demand is very dependent on the design of buildings, even for one location. Moreover, solar radiation on a horizontal surface is not particularly relevant to solar geometry in winter. That falling on a south-facing vertical surface, or a steeply sloping surface, is more likely to displace fossil fuels for heating. So, for example, one might expect the incident irradiation from September to May in the Western Isles to be greater than that from November to April in London. Overall, although the idea of such a ratio as a ready reckoner of potential may appeal, it would need to be carefully devised to eliminate an erroneous southern bias.

Brighton Breakthrough

The analysis presented at Brighton was subject to assumptions for a simple theoretical model for a small dwelling, which was well insulated. It was such a powerful dispeller of previous myths that some numerical detail is required. The house was not designed, but rather assumed to have a low specific heat loss or heat loss coefficient, as well as a particular size and efficiency of solar collection.

1.2(1) For a representative UK location, this suggested a contribution of about 2500kWh over a heating season from October to April. Residual or net space heating loads would then be in the order of 3000kWh.

1.2(2) These numbers are important in that they signify a considerable level of energy efficiency. Most householders would be pleasantly surprised if they found that they had only used 3000 units for heating in a year. They might also be surprised that their bill could have been more than eighty per cent higher but for the contribution from the sun.

By assuming an energy-efficient model, the analysis was both forwardlooking and avoided southern locations being deceptively favoured. Although a poorly insulated model would have lengthened the heating season for all locations, an efficient model moves towards eliminating space heating entirely. It is also important to emphasize that all the values used are reasonable and could translate into built form. The house might, for example, have a relatively long and high south façade, with a mono-pitched roof sloping down to the north across a relatively narrow depth.

1.3 This is an archetypal passive solar form. Roughly half of this might be either collector or window, with minimal glazing elsewhere and impressively low U-values or thermal transmittance coefficients for all opaque surfaces. Alternatively, if conceived as part of a terrace, the U-values could afford to rise a bit. This kind of solar dwelling could match the assumed heat loss coefficient.

The results showed higher solar savings with increasing latitude. For example, at the most northerly latitude of Lerwick in the Shetland Isles at 60°N the solar contribution was nearly four times greater than in the most southerly location of Messina in Sicily at 38°N. Even within the UK, Lerwick’s solar input was nearly 60 per cent greater than that for London

1.4(1). The explanation for this surprising conclusion was the better usefulness of solar energy at high latitudes due to the greater, longer and flatter profile for heating demand. This more than compensated for the slightly lower solar radiation levels, the reduction, as already stated, being a lot less marked on vertical surfaces than on tilted or horizontal ones. Altitude was also relevant. For example, at Eskdalemuir in the Scottish Borders at 250m, the solar contribution was predicted to be marginally greater than for Lerwick at sea level, while the residual demand for space heating was also somewhat higher

1.4(2). Later work, which examined the relative climatic suitability for solar space heating in different parts of Scotland (MacGregor and Balmbro, 1984), found that the best location was in the north at Wick (58.4°N), while the worst was in the south at Greenock (55.9°N), the difference being 45 per cent.

In Norway, Olseth and Skartveit (1986), using another analytical method known as ‘F-chart’, investigated the relative performance of an active solar heated building in different locations. Again it was found that solar savings are generally greater at higher latitudes, for example 10 per cent higher at Tromso (69.7°N) compared with Oslo (60°N). An earlier study in North America (Duffie, Beckman and Dekker, 1977) also concluded that solar savings...

Table of contents

• Cover
• Title Page
• Copyright
• Contents
• List of Boxes
• List of Figures
• Preface
• Acknowledgements
• Chapter 1. North Sun Context
• Chapter 2. Multiple and Added Solar Value
• Chapter 3. Environmental Comfort and Well-being
• Chapter 4. Adaptive Control
• Chapter 5. Passive Control
• Chapter 6. Machine Control
• Chapter 7. Green Solar Future
• Picture Credits
• Index of Solar Buildings and Projects
• Index of People, Practices, Institutions, Etc.
• Index of Defined Terms and Products
• Index of Acronyms and Abbreviations