utilization of passive solar technology

Lecture 3:Utilization of Passive Solar Technology

Lecturers:

Syafaruddin & Takashi Hiyama

[email protected]

[email protected]

Time and Venue:

Wednesdays: 10:20 – 11:50, Room No.: 208

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http://www.cs.kumamoto-u.ac.jp/epslab/APSF/

Contents:

1.1. IntroductionIntroduction

2.2. PrinciplesPrinciples

3.3. Technical DescriptionsTechnical Descriptions

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3.1 Definition3.1 Definition• Terms• Key Figures3.2 System Components3.2 System Components• Transparent Cover• Shading Device• Absorber and heat storage3.3 Functional Systems3.3 Functional Systems• Direct Gain Systems• Indirect Gain Systems• Decoupled Systems• Sunspaces

1. Introduction1. Introduction

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•‘Utilization of Passive Solar Energy’: first introduced in the 70’s of the last century. •In that time, "adding auxiliary energy" was used to clearly distinguish between passive & active solar energy applications If auxiliary is used, the system is referred to hybrid systemshybrid systems•Definition in a more realistic and precise manner (Recently): passive solar systems convert solar radiation into heat by means of the building structure itself, i.e. by the transparent building envelope and solid storage elements•Characteristic of passive solar architecture: “use of the building envelope as absorber & the building structure as heat store”

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2. Principles2. Principles Main energy flows within a building

Energy is primarily supplied by means of space heating systemsheat is created by

people, lighting and household appliances (internal heat gain)

passive solar heat gains, such as heat created by transparent surfaces (passive solar energy utilization)

Thermal mass is also capable ofabsorbing and intermediately storing heat in case of overheating. Heat is only released if the thermal mass becomes warmer than the room temperature.

Heat losses or heat gains (depending on the ambient temperature) are due to the heat conductivity of the building envelope (i.e. transmission).

to maintain a certain air quality and to prevent the system from exceeding the prescribed levels of carbon dioxide (CO2) and other harmful substances, air humidity and certain odours.

Passive solar energy utilization is based on the absorption of short wave solar radiationshort wave solar radiation through :

building interior (solar radiation penetrates through the transparent external structural elements)

building envelope

Mechanisms:

The concerned structural elements are warmed up by the absorbed solar energy

The energy is released back to the exterior by convection and long wave radiation

The quantity of absorbed solar energy of surfaces, exposed to radiation, depends on

their orientation

Shading equipment

absorption coefficient of the concerned absorber surface

The quantity and timing of released energy are determined by:

thermal conductivity

density

the specific thermal capacity of the absorbing material itself

and the material placed behind

As well as by the difference to surrounding temperature.

The seasonal effects of passive solar energy utilization seasonal effects of passive solar energy utilization can be further intensified by:

an appropriate orientation of the concerned surfaces or shading devices

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Continue…Continue…

3. Technical description3. Technical description3.1 Definition

Terms:Terms:The translucence of walls is often described by the terms opaque,

transparent, and translucent, as well as by solar aperture surface.

• The opaque building envelope is not permeated by light and includes, for instance, brick walls, or a roof covered with tiles.

• Transparent and translucent parts (e.g. windows) of the building are permeable by solar radiation In general, the word transparent means clear, whereas translucent parts of the building can not be seen through.

In terms of solar energy utilization, the word transparent is also used to describe external parts of the building that can be seen through but that are not clear, in order to express their permeability not only by visible light but also by other components of the solar spectrum.

The term solar aperture surface refers to the translucent envelope surface that is suitable for solar energy utilisation.

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Key figures:Key figures: Transmission coefficient

Secondary heat flow

Energy transmittance factor (g-value)

Diffuse energy transmittance factor (diffuse g-value)

Thermal transmittance coefficient (U-value)

Equivalent thermal transmittance coefficient (equivalent U-value)

Transmission losses

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3. Technical description3. Technical description3.1 Definition

Key figures…cont.

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Secondary heat flow factor (qSecondary heat flow factor (qii):):•indicates the share of global radiation Gg which is absorbed by a structural element and re-radiated into the building in the form of long wave radiation and convection •Transparent elements (glazing) also warm up a little by the absorbed incident radiation and thus also present a secondary heat flow

Transmission coefficient (Transmission coefficient (ττee) ) :•Indicates the share of global radiation incident on the irradiated structural element, which is transmitted through the glazing into the building as short wave radiation•It also considers the invisible wavelengths of solar radiation•If the transmission coefficient refers to the vertical radiation incidence, it is designated as τe*

Key figures…cont.

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Energy transmittance factor (gEnergy transmittance factor (g--value):value):•In addition to the energy supplied by radiation transmission (i.e. in addition to transmission coefficient τe), the g-value (g) or energy transmittance factor also includes the secondary heat flow qi•It has been defined for a vertical radiation incidence vertical radiation incidence and the same temperatures on both sides of the structural element For transparent building parts (glazing):it consists of the transmission coefficient τe*, assuming a vertical radiation incidence, and the secondary heat flow qi

qin represents the heat flow added to the structural elementGg the global radiation

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Diffuse energy transmittance factor (diffuse gDiffuse energy transmittance factor (diffuse g--value):value):Depending on time and season, solar radiation strikes the transparent building elements from very different angles. On average, solar incidence on transparent surfaces is thus not vertical.Furthermore, moderate climate is characterized by a high share of diffuse radiation, amounting to about 60 % of the total incident solar radiation and presenting an average incidence angle of about 60o. The diffuse g-value considers the decreased energy transmittance factor or g-value in case of vertical incidence, amounting to about 10 % . When compared to the conventional g-value g, the diffuse g-value, allows for more realistic results.

Key figures…cont.

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Thermal transmittance coefficient (UThermal transmittance coefficient (U--value):value):•Definition: a measure of the heat that is transmitted from the front side of a façade to the inside, assuming an area of 1 m2 and a temperature difference of 1 K• It consists of heat transfer from air on one side of the element, thermal conductivity within the structural element and thermal transmission from the other side of element to the air•In case of double-glazing, heat is transmitted by long wave radiation and convection between the two glass panes•For windows, we distinguish the UG-value, which solely refers to the glazing, and the UW-value, which also considers heat losses of the window frame, and thus refers to the entire window

Key figures…cont.

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Equivalent thermal transmittance coefficient (equivalent UEquivalent thermal transmittance coefficient (equivalent U--value): value): •Definition: a measure which describes the difference between the specific thermal losses of a structural element and its specific heat gain by solar radiation. •Like the U-and g-values, it also depends on the radiation incidence on the transparent surface and the dynamic behavior of the building located behind. •For its determination only thermal gains during the heating season must be taken into account, as overheating of rooms due to the solar radiation on glazed surfaces is not desirable.•A negative equivalent U-value indicates that the heat gained by a transparent surface exceeds its thermal transmission. •The approximate estimation of the equivalent U-value (Ueq) by means of the UW-value, referring to the entire window (including frame), the g-value (energy transmittance factor) and a correction factor SW for window orientation.

Key figures…cont.

•The correction factor SW varies between 0.95 for north facing, 1.65 for east and west facing, and 2.4 for south facing orientation

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Transmission losses:Transmission losses:The thermal losses of a building consist of:*ventilation and infiltration losses *transmission losses Transmission losses are calculated by means of the U-values of the respective surfaces (i.e. of surfaces An) and the temperature difference between internal room temperature θi and the corresponding external temperature θe of all structural elements of a house:

Key figures…cont.

However, transmission losses must not be mistaken for the transmission coefficient τe of transparent structural elements.

Passive solar systems may consist of:

• Transparent covers

• Shading device

• Absorber and heat storage

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3. Technical description3. Technical description3.2 System Components

Transparent covers

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Total energy transmittance factor of an average thermal insulation double-glazing

Only one part of the incident solar radiation is transferred into theinterior, whereas the remaining part is reflected from the outer pane surface

The radiation share directly transferred into the interior through both panes is indicated,in proportion to the radiation incidence on the outer pane surface, by the transmission coefficient transmission coefficient ττee

Another portion of the incident solar Another portion of the incident solar radiation radiation is absorbed by the glass panes and heats up the gap between the two panes, and thus leads to further heat transmission into the interior by long wave radiation and convection

The g-value or energy transmittance factor indicates the ratio of total heat transferred into the interior and the incident radiation

Transparent covers…cont.

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Transparent covers (such as windows) serve to transmit a maximum share of solar radiation to the interior and ensure at the same time utmost insulation from the outsideTypically, these two properties are expressed by the g-value (energy transmittance factor) and the U-value (thermal transmittance coefficient)

Good transparent covers are characterized by high g-values and low U-values

Improvement:*Noble gas fillings between the two glass panes, characterized by low thermal conductivity, low specific thermal capacity and high viscosity, help further reduce the thermal transmission by convection between the two glass panes. *optimum adjustment of the pane spacing ensures the lowest possible U-values.

In the past: the single glass panes and insulating glazing offered very high g-values, but very high, and disadvantageous U-values

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Optimum spacing between panes and thermodynamic properties of some window filling gases at 10oC

Another approach:Low ε-coated double and triple glazing with noble gas fillings and panes with infrared reflecting coating ensure both: low U-values (thermal transmittance coefficient) and high g-values (energy transmittance factor).

Reasons: low emission coefficient (ε) -coatings help reduce thermal losses due to radiation exchanges in between the two panes. * long wave radiation from originally 0.84 to 0.04* short wave radiation these coatings are highly transparent

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By developing glazing developing glazing with a high energy transmittance factor g, and transparentthermal insulation material (TI) we obtain transparent covers that offer both a high energy transmittance and good thermal insulation


Table 3.2 illustrates some examples of g and U-values of some typical glazing types and aselection of transparent insulation systems. To indicate the energy transmittancealso the diffuse g-value is considered.


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Table 3.3 shows the equivalent U-values corresponding to different glazing types

By selecting state-of-the-art south facing double-glazing with thermal insulation,heat losses can be nearly compensated; triple glazing can achieve energy gains. The heat gain of a high-class north facing triple glazing can even exceed its heat transmission.


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However, the diffuse g-values indicated in Table 3.3 only apply to the glazing itself.For window calculation the frame thus needs to be deducted from the

window surface*For large-surface windows the U-value of a window Uw includes a 30 % frame surface*For smaller windows the Uw-value needs to be recalculated by means of the thermal transmittance coefficient (U-value) for both frame and glass pane, and additional thermal losses due to connecting sections have to be considered

The g-value (energy transmittance factor) of a glass pane is additionally reduced by *dust on the glass pane FD *possible fixed shading FS *flexible shading FCEven for frequently cleaned surfaces, due to dust, a reduction of theg-value by 5 % has to be assumed. The value needs to be further reduced to consider the inclined radiation incidence. This factor is taken into account by the diffuse g-value in Table 3.2 and Table 3.3.


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The solar heat generated within a defined period of time within area QS is thus calculated by multiplying the solar global radiation incident on window Gg,t,a, g-value and reduction factors, such as fixed and flexible shading (FS and FC) (Table 3.4 and Table 3.5), contamination FD, and frame section FF (Table 3.6):

Shading devices

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•By an appropriate building design, e.g. by balconies and projections, adequate shading protection from high-angle sun in summer can be provided without incurring any additional costs. •The advantages of such fixed shading devices are simplicity and permanent function, as the devices lack moving parts which would require special control. •However, they are applied most easily in a new building, where they can be incorporated into the original design, and should be south facing to ensure good shading in summertime and high irradiation into the building by the low-angle sun in winter•Even in summertime orientation to the east and west provides for high irradiation of the building by the low-angle sun, whereas in wintertime irradiation for these orientations is low •However, fixed shading devices reduce the efficiency of passive solar energy utilization, as they also provide shading in-between seasons (in spring and fall), when space heating systems are still required.

Shading devices…cont.

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Shading of transparent building surfaces by roof overhangs

single family house

multi family house


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Shading of buildings thus depends on the following parameters or factors:

For a definition of the relevant angles:

Shading by the horizon Fh: determined by the solar position plot Shading by projecting structures: overhangs Fo and side overhangs (fins) Ff

Shading factor FS covers the entire shadingit is composed of the horizon shading factor Fh, the shading factors of overhangs Fo and side overhangs Ff. “Dynamic building simulations allow for a more precise determination of the total shading of a building than this simplified equation”


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Besides the described fixed shading elements, also adjustable shading devices adjustable shading devices are used for passive solar system control*If the solar heat gain exceeds, for instance, the heat demand of a living space to be covered by solar resources, the solar aperture surface can be shaded to prevent overheating. *External shading elements, such as window blinds and shutters, re-transmit the absorbed radiant heat to the ambient air, and are thus often more efficient than internal shading equipment. *However, internal shading equipment (such as shutters and drapes) does not have to be weatherproof and is thus less demanding in terms of design.


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Shading by internal and external window blinds (θe

ambient temperature, θi room temperature)

The figure shows furthermore, that internal window blinds can only slightly reduce the inside temperature, whereas external blinds can reduce the inside temperature by several degrees Kelvin. In the present case, no additional cooling is required when using external window blinds.

Room temperature of a building calculated by dynamic building simulation for both cases;*without shading devices *two different shading methods One week in summer and ambient temp. θe (12- 27) oC.

Assumption: Assumption: active air cooling is assumed for room temperatures above 26 oC; room temperature θi will thus not increase beyond this value

Shading devices…cont

In contrast, reliability is considerably enhanced with shading systems incorporated into the glass pane. These systems include the following operating principles:

− Thermotropic glazing becomes opaque at defined outside or system temperatures, as molecules tend to accumulate to a pane-incorporated gel layer.

− Electrochromic glazing is characterized by a special coating which converts from transparent to opaque at a defined voltage.

− Glazing covered by holographic foils reflect the irradiation from the high-angle sun, so that sun-rays incident at small angles reach the absorber without any obstacle.

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Absorber and heat storage

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Absorber and heat storage are individual components in active solar systemsactive solar systems, they are integrated into the building structure of passive systems.Within direct gain systems, the room envelopes with solar radiation exposure, serve as absorber surfaces.Passive solar systems should thus offer wellwell-absorbing outer surfaces and a heat-storing building structure which is well-adapted to the solar system

The "classic" passive energy system is not equipped with any control. The thermal mass of a house, which is heated up by solar radiation, releases the heatback into the internal space with a certain time lag and reduced temperature withoutany user intervention.

It is thus essential to prevent passive accumulators from overheating the rooms.*For this purpose, the time lag and heat flow reduction by passive storage need to be known factors. *Also, in most cases, additional (active) shading devices need to be provided to reduce energy absorption in summertime.

Absorber and heat storage…cont

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Indirectly heated thermal mass (e.g. unheated internal walls) Indirectly heated thermal mass (e.g. unheated internal walls) can only be usedsensibly if the corresponding room temperature variations are permitted. •In case of high room temperatures, heat is slowly absorbed by thermal mass which isgradually heated up by the space. •If, by contrast, the room temperature falls below the thermal mass surface temperature, the stored heat is released back into the space.

The established heat flow depends on:*temperature (q) difference between the warm and the cold accumulator, *specific thermal capacity cp, *density ρSM

*thermal conductivity λ of the storage medium*heating and heat dissipation time t. Within a very short period of time, for instance, the accumulator will only heat up at the surface, and the absorbed energy quantity is thus only little.

The heat flow is calculated by Fourier’s (one-dimensional) law on heat conduction:

The heat flow into and from the storageElement:

Absorber and storage…cont.

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Within 24 hours:the temperature varies by 6oC on one room side, whereas the stored and released energy amounts to 0.076 kWh/(m2d)

Temperature flow within an internal concrete wall (storage wall) exposed to radiation and varying temperatures on one side within 24 hours

•The temperature only varies significantly up to a wall thickness of approximately 15 cm. •An increased wall thickness does thus not enhance the heat storage capacity.


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In most indirect gain systems:In most indirect gain systems:*only the outer wall is used for heat storage, which is thus of solid design *outer wall surface serves as absorberFor this purpose the surface is either painted in black or covered with black absorber foil

Only within decoupled systems:•absorber and accumulator are separate components; whereas black or selectively coated sheet metal serves as absorber medium.•The heat carrier is transferred to the accumulator by means of a channel or a more sophisticated medium. •The accumulator itself may also be part of the building structure, for instance, if it is designed as hollow ceiling or double wall masonry.•Rock storage, however, are not of double use, as they are not part of the building structure.


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Dynamic building simulations permit the determination of energy requirementsfor heating QH by means of *building heat losses QL reduced by usable energy from solar radiation QS and internal heat Qi (i.e. heat created by people and household appliances) as well as utilization factor η.

The utilization factor η for the available solar energy QS, and internal heat gains Qi

A ratio of heat gain to heat loss γ:

Depending on their form and arrangement:

• Direct Gain Systems

• Indirect Gain Systems

• Decoupled Systems

• Sunspaces

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3. Technical description3. Technical description3.3 Functional System

Direct Gain Systems

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•Solar radiation penetrates into the living space through transparent external surfaces and is converted into heat at the internal room surfaces.•Room temperature and room surface temperature change almost simultaneously.

Typical direct gain systems are regular windows and skylights

Direct Gain Systems…cont.

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Characteristics:*simple structure, •low control requirements •low storage losses, as radiation energy is on the spot convertedinto heat, inside the living space. Disadvantageous:*poor ability to react to phase shifts between radiation and internal temperature may be disadvantageous.

Other properties: Direct gain systems can only be controlled by shading, since the heat which is re-radiated into space by thermal mass cannot be influenced. Hence, to ensure profitable utilization of solar gains, additional heating systems with low inertia are required

Advantageous: Advantageous: *profitable if radiation and heating demand occur simultaneously, as in many office buildings. *may also be combined with daylight systems in order to save the energy required for lighting. *suitable to complement indirect gain systems which respond more easily to the phase shifts between incident radiation and heating demand.

Indirect Gain Systems

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Indirect gain systems (solar wall):*solar radiation is converted into heat on the side of the storage element opposite to the living space.*Energy migrates to the storage element’s inner surface (room side) by heat conductivity and is released into the room air. *There is thus a certain phase shift between solar radiation and internal temperature, which is influenced by the storage element material and its thickness.

Indirect Gain Systems…cont.

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Characteristics:*simple structure, *phase shifted room heating and lower room temperature variations in comparison to direct gain systems.Disadvantageous:higher heat losses to ambient

Solar wall systems or indirect gain systems are suitable to complement directgain systems;

Reasons: when combined, they extend heat distribution into the living space. Combinations of both systems are particularly appropriate for living spaces with continuous heat demand.

•Heat supply can only be regulated by appropriate shading devices•Once the heat has been absorbed by the storage element, heat transfer into the living space can no longer be controlled•For systems supported by convection, also the inner side of the transparent cover needs to be cleaned as room air and heated air tend to mingle


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Opaque thermal insulationtransparent thermal insulation

Transparent thermal insulation:Transparent thermal insulation:Solar wall systems equipped with transparent thermal insulation (TI) have been developed to enhance passive solar energy gains. They are a special variant of indirect gain systems, as they enable the effectiveutilization of passive solar systems in Central Europe thanks to the use oftransparent heat insulators.


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Opaque thermal insulation:Opaque thermal insulation:•Only very little of the solar radiation incident can be utilized •During absorption of solar radiation the outer surface may heat up; however, due to the low thermal conductivity of the insulation layer, also in case of considerable temperature differences (between the inside and the outside of the space) only very little heat will penetrate to the inside.

Transparent thermal insulation:Transparent thermal insulation:•a large amount of solar radiation penetrates through elements is converted into heat when striking the black coated element (absorber)•Due to the high thermal resistance of the insulating material, a large amount of heat is transferred into the storage wall.


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Temperature distribution of a system consisting of glass –transparent thermal insulation (TI) – air – absorber –concrete wall on a cold and foggy winter’s day

•To provide for good transfer of the heat created at the absorber (useful heat) and to avoid excessive maximum temperatures, the wall behind the transparent thermal insulation material (TI) must have a high thermal transmission coefficient and good storage properties. •However, these properties will result in very low thermal insulation. •The total U-values of transparent thermal insulation (TI) are thus often higher than those of a solely insulated wall. •At night time, when thermal mass has cooled down, the wall will have higher losses than a solely insulated wall. •However, for well-designed walls with thermal insulation heat losses are in most cases overcompensated by heat gains; •The equivalent U-values (Ueq) (including solar gains) are lower or even negative (net heat gain). •In the example (Figure): the U-value amounts to 0.527 W/(m2 K) and the equivalent U-value (Ueq) to 0.267 W/(m2 K) on this unpleasant day. Notes: U-value (thermal transmittance coefficient) and

the equivalent U-value (Ueq)


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Solar systems with convective Solar systems with convective heat flowheat flow (A further variant of indirect gain systems)These systems do notrequire any shading, as, in summertime, the hot air between absorber and accumulator is evacuated to the outside.

Decoupled Systems

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•Decoupled systems convert the incident solar radiation on an absorbing surface, thermally insulated from the internal space •Subsequently, solar heat is transferred to a heat accumulator via a channel system, using air as heat carrier. •The heat accumulator can either be incorporated into the building structure or be itself an individual technical component (or a combination of both). •Hollow ceilings and double wall masonry are examples of accumulators incorporated into the building structure, whereas rock and water storage tanks are systems that are independent of the building structure.

Thermally decoupled solar system attached to the building envelope

Decoupled Systems…cont.

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Characteristics:Characteristics:*good controllability Thanks to the thermal insulation between absorber and the internal space *at night heat losses are very lowDisadvantageous: Disadvantageous: *these systems present high construction costs* a propensity towards defects (e.g. leakages) and high absorber temperatures

Advantageous:Advantageous:*Thermally decoupled systems are most suitable to compensate considerabletime lags between solar radiation and heat demand*They also result advantageous if separate heat accumulators are already available or if they can be easily integrated into the building structure

Sunspaces

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•Sunspaces are another variant of functional systems. •Most popular are unheated sunspaces, whose connecting doors to the internal living space are left open if heating is needed and the temperature inside the adjacent sunspace becomes warmer. •Furthermore, sunspaces of two or more stories also serve for ventilation of houses.• In wintertime the minimum temperatures amount to 0oC, whereas in summertime heat needs to be evacuated to the outside to avoid overheating (temperatures above 50 oC are possible). •For this reason slanting windows should be avoided and the roof should be well-insulated. •Also, orientation toward the east and west is unfavorable, as the incident solar radiation will be very little in winter, and in summer shading can only be provided by window blinds, but not by roof overhangs.

Sunspaces…cont.

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Operation principle of a sunspace

Characteristics:Characteristics:•During the heating season a well designed sunspace, with optimum control, supplies the house with just as much or a little more energy than it receives from the house. •Besides the utilization of passive solar energy, unheated sunspaces also cut the heat load of a building, as the system wall – sunspace –wall usually presents a lower U-value (thermal transmittance coefficient) as the outer wall. •A heated sunspace, however, will result in higher heat losses.

Sunspaces…cont.

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Even shaded sunspaces provided with roof overhangs are often overheated in summertime.

Illustrations: Illustrations: the temperature curves of the sunspace (θss), the living space temperatures (θi), the ambient temperatures (θe) and the floor (θfl) and ceiling temperatures (θce) in a house equipped with a floor heating system on three nice days in summer. ***In spite of a high ventilation rate, temperature rises to above 40 oC. However, the maximum temperature in the adjacent living space only amounts to 30 oC***

Temperature flow of a sunspace in summertime

utilization of passive solar technology

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