Table of Contents
What is a Passive Solar Home Design?
Passive solar design is a specific variation of passive housing which looks to develop the building site, climate and construction materials to optimise solar energy use. A comprehensive design will maximise solar heat gain, convert solar energy through a PV system and minimise heating where it becomes detrimental (in order to reduce cooling costs).
When looking to add solar features to new build design, or as part of a retrofit, keep in mind that energy efficiency is the most cost-effective strategy for reducing heating and cooling bills. A functional design will cut heating and cooling loads through the use of energy-efficient strategies and then meets those lessened demands through the generation of electricity generated by solar panels.
As more modern houses have more effective building envelopes than those of previous generations it is important that any design avoids the temptation to oversize south-facing windows, ensuring that they are appropriately shaded to prevent situations of overheating from arising. Should this arise the cooling loads would be increased throughout the hotter periods of the year.
Renewable Energy Product Audits
Due to energy efficiency being the key focus of any solar passive design, it is recommended that you choose experts who have ample experience in energy-efficient house design and construction so that they can guarantee your home’s energy efficiency will meet the standards expected by both you and the Passive House Standard.
We at UK Alternative Energy are experienced MCS accredited Renewable Energy Specialists and have extensive experience in renewable energy projects, including Passive House projects. For more information, you can contact us via email at info@ukalternativeenergy.co.uk or call 01522 803 854.
How does a Passive Solar Home Design work?
The core concept of a passive solar home build revolves around the sun shining through south-facing windows and can retain this ‘thermal mass’ through materials which store heat energy. The percentage of a buildings heating load which is met by a solar passive design is known as the ‘passive solar fraction’. This fraction greatly varies depending on the surface area of the property’s window glazing and the amount of construction material which can hold thermal mass.
It is important for prospective owners to understand that the ideal ratio of thermal mass to glazing varies between different climates. Comprehensive solar passive designs also make it possible to provide daylight all year round and comfortable temperatures throughout the cooling season by operating night-time ventilation.
The fundamental items which need to be included within a passive solar design include:
- Solar Orientation: When designing a new passive solar build, the most vital consideration needs to be the position of the sun in the sky at various times of the year; this being the path that the sun takes through the sky during the day, alongside the total and seasonable solar insulation levels available. This calculation must account for the average level of sunlight expected in the local geographical area and the number of expected cloudy days. With this calculation in hand, you can design a building to allow for high levels of thermal gain when desired, and also for restricted heat gain when it is unwanted. The fundamental means by which this is achieved is through the placement and sizing of windows present on a property. Alongside these factors, the orientation of the build’s walls should be reviewed, as often is the case that straight walls are not the ideal answer when looking to maximise solar energy gain. When addressing the correct position for a design’s windows, the general rule within the UK is for properties to face within 30 degrees of true south and ought to not be shaded during the heating season by other buildings or trees between the times of 9.a.m. to 3.p.m. on a daily basis. During the spring, fall and cooling season, the windows should be adequately shaded to prevent internal overheating.
- Thermal Mass Materials: Whilst passive solar designs share many similarities to other well-insulated buildings, using many of the same materials, they look to operate these materials in a different manner than is done through conventional designs. The most commonly used materials for these purposes are stone, concrete, brick, tile and adobe. Interestingly other thermal mass materials such as water and phase change products are more efficient in storing heat, however, stonework has the added benefits of being a structural finish material as well. In order for these materials to function properly as thermal mass retainers, they must be left bare, with no obstacles blocking them from exposure to direct sunlight. Do factor in the thermal mass inherent in home furnishing and drywall may sufficiently meet the needs of a build, removing the need for additional thermal storage materials. Nearly all passive solar builds make extensive use of double, or triple glazed windows and attachable window quilts or shades/shutters. Referring back to the key focus of the passive design, it is imperative to ensure solar gain is maximised and that the glazing chosen does not have a detrimental effect on this gain. An additional factor that may be overlooked is the unnecessary use of skylights, with there being a misconception that skylights are an ideal inclusion in passive builds. The problem with skylights being included is they maximise heat gain at the wrong times of the year and augment heat loss at the wrong times as well. Often times becoming an unintentional heat funnel which emphasis hot or cold temperatures rather than controlling them. Insulated light tubes could be used as a suitable replacement if the design is thought to require additional efficiency items.
- Distribution Mechanisms: When solar heat has been introduced into the property it is essential that this heat can be transferred from the collection point and be allocated to areas where it is needed via convection, conduction or radiation. In smaller builds, small fans and ventilation blowers may suffice in carrying out the heat distribution. Convection is the process whereby heat is transferred through the air or water, and this is the default process of distribution used within passive solar homes to transport air from warmer areas to other rooms in a timely manner. Convective heat transfer can be advantageous or detrimental depending on how well a ventilation system operates. Uncontrolled air infiltration from poor weatherisation can result in a loss of up to 40% of heat. Fortunately, the strategic placement of operable vents and windows can promote greater convection and cross-ventilation. Conduction occurs when heat energy travels between two adjoining objects in direct contact with each other, such as a sun-heated floor warming the soles of your feet. Due to this process, colder areas within a build will naturally attract heat energy to compensate for the low temperatures. Radiation is the main source of heat transfer, with this being the way in which the Sun transmits energy to the Earth. Solar radiation predominantly occurs through the roof, walls and windows. Thermal radiation is the process which transfers heat energy from a warmer surface to a colder one. Feeling heat transmitted from a wall or window would be a tactile example of this in action. Darker colours absorb more heat than lighter colours and when designed effectively can provide travel routes for a thermal mass movement in passive solar buildings.
- Control Strategies: Appropriate roof overhangs can provide adequate shade from vertical south windows during the summer months. These overhangs can be attached either internally or externally. From insulated cellular shades to window films and exterior awnings, there are several options to choose from and every individual build may benefit from a specific overhang. Other control mechanisms can include electronic sensory devices, such as an automated thermostat which can signal the ventilation system to turn on at specific internal temperatures, with operable vents and dampers which can promote or restrict heat flow.
What is the difference between a Direct, Indirect and Isolated Gain Passive Build?
There are three types of approaches which can be used in passive systems, these being; direct gain, indirect gain and isolated gain. The aim of all passive solar heating systems is to capture solar energy within a build’s elements and release that heat during periods where the sun is not shining, and internal temperatures are dipping.
Direct–gain designs allow sunlight to enter the house through the south-facing windows to strike the stonework floors and walls, which absorb and store the heat energy. During the night, when temperatures drop the thermal mass will release heat energy to ensure stable temperatures are maintained. Direct gain systems are the most efficient of the three approaches, making use of 60 – 75% of the solar energy exposed to the windows.
It is not uncommon for some builds to utilise water-filled containers positioned inside the living space to absorb and store solar heat. Whilst the greatest advantage of using water storage is that it can retain twice as much heat energy as masonry materials per cubic foot of volume, water thermal storage requires carefully designed structural support.
This option can be preferable for retrofit projects looking to improve heat retention so long as the structure can withstand the weight load. A replacement of masonry materials would be far more costly and would be even less effective due to it being only partial construction.
Indirect-gain passive solar homes place their thermal storage between the south-facing windows and the living spaces. The thermal mass absorbs the sunlight that strikes it and transfers the energy to the living spaces via conduction. An indirect gain system is likely to use between 30 – 45% of the sun’s energy the thermal mass has been exposed to.
There are two types of indirect gain systems, these being thermal storage wall systems and roof pond systems. For a thermal storage wall system, the mass is located immediately behind the south-facing glass and has attached vents positioned at the top and bottom of a thermal storage wall with allows heat to convect from between the wall and glass into the living space. During the night-time, the vents are closed, and heat radiates from the walls to heat the living spaces.
Roof pond systems have water stored in large plastic or fibreglass containers covered by glazing, with heat energy directed from the above-heated water into the area below.
Isolated-gain systems have their key parts separated from the main living spaces of a house, with a prime example being a sunroom which retains the thermal mass in the air of the room. Sunlight enters the building through conduction of a shared mass wall in the rear of the sunroom, or by vents which allows for access between the sunroom and the living space.
An isolated gain system will utilise 15 – 30% of the sunlight striking the glazing into heating the necessary living areas.
Supplementary Solar PV and Battery Systems
With passive solar designs making great use of the heat energy transferred naturally by the sun to greatly reduce annual heating costs, another aspect and prominent tool which makes use of solar power are comprehensive Solar Photovoltaic Systems with a connected Solar Battery, which convert the suns energy into electricity and stores it for later domestic or commercial use.
Using a solar PV system with a battery will allow an owner to make even further overtures in cutting their energy costs and eliminating their heating costs completely. We, at UK Alternative Energy, have carried out numerous domestic and commercial solar panel installations for individuals and businesses looking to make the most out of solar energy. Combing a solar passive design alongside such a system would result in noticeably low energy bills and is ideal for retrofits which do not aim to replace their roof materials with more solar passive friendly tiles.
