Yeang K., Woo L. Dictionary of Ecodesign: An Illustrated Reference

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Thermal envelope house An architectural

design (also known as a double envelope house),

sometimes called a “house-within-a-house”, that

employs a double envelope with a continuous

airspace of at least 6–12 inches (15–31 cm) on

the north wall, south wall, roof, and ﬂoor,

achieved by building inner and outer walls, a

crawl space or subbasement below the ﬂoor,

and a shallow attic space below the weather

roof. The east and west walls are single, con-

ventional walls. A buffer zone of solar-heated,

circulating air warms the inner envelope of the

house. The south-facing airspace may double as

a sunspace or greenhouse. See also:

Double

envelope

Thermal ﬂywheel effect Also known as ther-

mal momentum. The property of a material to

remain at a given temperature; related to the

building’s mass and the thermal conductivity of

the materials connecting the mass to the interior

space. It is generally applied to buildings or

construction materials. This is distinct from a

material’s insulative value. A building’s average

internal temperature can remain stable much

longer due to this effect. For example, the ther-

mal mass of a house is a measure of its capacity

to store and regulate internal heat. Buildings

with a high thermal mass take a long time to

heat up, but also take a long time to cool down.

As a result they have a very steady internal

temperature. Like a ﬂywheel, the thermal mass

can store and even out ﬂuctuations in tempera-

ture. Buildings with a low thermal mass are very

responsive to changes in internal temperature—

they heat up very quickly, but they also cool

down quickly. They are often subject to wide

variation in internal temperature. Materials with

a higher to lower ﬂywheel effect include: packed

earth, brick, water, hardwood lumber, softwoods,

steel, insulation, air, and aluminum.

Thermal inertia Tendency of a massive material

to remain at a certain temperature.

Thermal mass Material that stores heat. It can

be a liquid or solid substance, and is character-

ized by a charge-up or dissipation time. Heat

storage and heat sink describe the same con-

cept. Liquid thermal mass tends to stratify. A

solid thermal mass’s ability to store and transfer

heat depends on the material’s speciﬁc heat

capacity, thermal conductivity, and convective

heat transfer. Architecturally, thermal mass heats

up slowly when the surface conditions are

warmer than the interior of the mass, and dis-

charges heat when the surface conditions become

cooler than the interior of the mass. Solar energy

can be used to heat the surface of a thermal

mass to increase the energy being stored. The

thermal mass averages the temperature and, in

effect, reduces the hot and cold peaks a room

would otherwise experience. Heating a thermal

mass with solar energy during the winter heats

Figure 75 The thermal ﬂywheel effect

238 Thermal envelope house

the mass that will radiate energy over a longer

period through the cooler night to maintain

warmer conditions in a room. Conversely, expos-

ing a thermal mass to cool nighttime conditions

during a warmer summer can lower the daytime

temperature in a room.

The use of thermal mass does not increase or

decrease the total energy available, nor does it

change the long-term heat loss or gain of the

building. However, it does help to reduce energy

demands in a home under mild heating and

cooling climate conditions. It also helps reduce

extreme temperatures within the home, main-

taining moderate temperatures inside the home

year-round.

Heat ﬂows from warm objects to cool objects

by way of conduction, convection, and radiation.

Conduction is transfer of heat through physical

contact, such as a thermometer under your

tongue, or the transfer of heat to the ground.

Convection is transfer of heat through a ﬂuid,

such as an ice cube cooling a glass of water.

Radiation is the transfer of heat through free

space, such as warming your hands on an open

ﬁre, or feeling the heat from the Sun’s rays.

Thermal masses use these three types of heat

transfer in a four-step process.

i) Heat is radiated to the surface of the mass by a

warmer object (such as the Sun, lights, people,

or equipment), and heat can be convected to

the surface via air circulation.

ii) Heat is then conducted from the warmer sur-

face of the mass to the cooler interior of the

mass, effectively “storing” heat in the mass.

iii) When the mass surface becomes warmer

than other objects surrounding it, the mass

radiates heat to these objects (meaning the

mass radiates heat back into the house) and

transfers heat to the room by convection

using the room air.

iv) Heat from the warmer interior of the mass is

conducted to the surface of the mass as the

mass cools, a reversal of step ii.

Thermal momentum See:

Thermal ﬂywheel effect

Thermal pollution Increased temperature of water

resulting from the disposal of waste heat from

industrial or power-generation processes. Ther-

mally polluted water can harm plants and animals

that cannot adapt to the increased temperature.

Thermal power plant Generic term for a facility

that generates electricity by using some energy

source to convert water to steam. The steam is

used to turn turbine generators.

Thermal storage Term used to describe var-

ious technologies that store heat, usually from

active solar collectors in an insulated repository,

for use later to heat space or hot water, or to

generate electricity. The thermal reservoir can

be kept at temperatures above or below that of

the ambient environment. Currently, there are

experiments to use molten salt as a medium to

retain a high-temperature thermal store for later

use in electricity generation. In a passive solar

building, thermal storage meets two needs: to

absorb solar heat quickly for use over the diur-

nal cycle; and to avoid overheating. There are

two basic thermal storage strategies using ther-

mal mass. i) Direct thermal storage materials,

such as concrete masonry or tiles, are placed

directly in sunlight so that intense solar energy

enters them quickly. ii) Diffuse thermal storage

materials are placed throughout the building.

They can absorb heat by radiation, through the

reﬂectance of sunlight as it bounces around a

room, and via air heated elsewhere in the

building, such as sunspaces and atria. Concrete

and masonry ﬂoors and walls exposed to direct

sunlight are probably the most common form of

thermal storage used in passive solar buildings.

Masonry materials have high thermal capacity;

their natural dark color aids in the absorption of

sunlight.

As water has high thermal capacity (about twice

that of common masonry materials), passive

Thermal storage 239

solar designers have experimented with a vari-

ety of water-storage containers, primarily built

into walls. Solutions include enclosing water

containers in seating boxes under south-facing

windows, or using water as an indoor feature

such as a large tropical aquarium, pond, or pool.

Trombe walls are also used for thermal storage

in passive solar structures. The wall is a south-

facing masonry covered with glass spaced a few

inches away. Sunlight passes through the glass

and is absorbed and stored by the wall. The

glass and airspace keep the heat from radiating

back to the outside. Heat is transferred by con-

duction as the masonry surface warms up, and

is slowly delivered to the building some hours

later. See also:

Thermal mass; Trombe wall

Thermal wheel Also called a heat wheel. Heat

exchanger rotating between two air ﬂows to

transfer heat from one to the other. It is usually

packed with heat-absorbing material such as

aluminum or stainless steel wool. The wheels

transfer energy from one air stream to another

or, for large boiler plants, from ﬂue gas to com-

bustion air. Thermal wheels can be installed

only when the hot and cold airstreams are

immediately adjacent and parallel. There are

two types of thermal wheel: one that transfers

sensible heat only; and one that transfers both

sensible and latent heat.

Thermodynamic cycle Process in which a work-

ing ﬂuid (water, air, ammonia) successively chan-

ges its state (from a liquid to a gas, and back to

a liquid) for the purpose of producing useful

work or energy, or of transferring energy.

Thermodynamics Description of heat movement.

Thermodynamics, ﬁrst law Energy can be

neither created nor destroyed.

Thermodynamics, second law When free

interchange of heat takes place, it is always the

hotter of the two bodies that loses energy and

the colder that gains energy.

Thermophotovoltaic cell (TPV) Device in

which sunlight concentrated onto an absorber

heats it to a high temperature; the thermal

radiation emitted by the absorber is used as the

energy source for a photovoltaic cell that is

designed to maximize conversion efﬁciency at

the wavelength of the thermal radiation.

Researchers currently have reservations about

its practicability and efﬁciency.

Thermosiphon 1. Natural convective movement

of air or water due to differences in density

caused by differences in temperature. In passive

solar design, a thermosiphon collector can be

constructed or attached to a house to deliver

heat by a continuous convective loop.

2. Common type of solar water heater that

uses collectors and circulating water, but is still

a passive system as no pumps are involved. The

storage tank is installed above the collector. As

water in the collector is heated and becomes

less dense, it rises into the tank by convection.

At the same time, cooler water in the tank sinks

into the collector, causing circulation through-

out the system. The storage tank is attached to

the top of the collector so that thermosiphoning

can take place.

Thixotropy Characteristic of a material that

allows it to stiffen or thicken in a short time.

When the material is shaken or manipulated, it

changes to a very soft consistency or a high-

viscosity ﬂuid. The materials are gel-like at rest

but ﬂuid when agitated, and have high static

shear strength and low dynamic shear strength

at the same time. See also:

Bentonite

Thomson effect Predicts the generation of

heat along a current-carrying wire that also has

a temperature gradient along it. Thomson’s

equation contains two terms: the nominal joule

240 Thermal wheel

heating or I

R loss; and the Thomson effect,

which quantiﬁes the amount of heat generated

by multiplying the current, the Thomson coefﬁ-

cient, and the temperature gradient as a function

of the length. See also:

Peltier effect; Seebeck

effect

Tidal electric power Power available from the

rise and fall of ocean tides to generate elec-

tricity. A tidal power plant works on the princi-

ple of a dam or barrage that captures water in a

basin at the peak of a tidal ﬂow, then directs the

water through a hydroelectric turbine as the tide

ebbs. There are two types of generating system:

tidal barrage, and tidal turbine. The barrage

system uses the water ﬂow on the ocean ﬂoor

(basin) to turn a turbine connected to an electric

generator. The tidal turbine is anchored to the

ocean ﬂoor and uses the large-scale persistent

ocean currents, often rip tides, as its source of

power. Both are experimental, expensive to install,

and may have adverse effects on water turbidity,

sedimentation, and marine life.

Tight building Structure designed to minimize

inﬁltration of air from the outside. This reduces

heating and cooling costs.

Tilt angle Degree at which a solar collector or

module is set to face the Sun relative to a hor-

izontal position. The tilt angle can be set or

adjusted to maximize seasonal or annual energy

collection. See also:

Photovoltaic module tilt

angle; Figure A.2 in Appendix 4: Photovoltaics

Time lag Term used in relation to release of

heat into a space after solar radiation has heated

a building’s material. See also:

Thermal storage

Time-of-use rate (TOU) Variable pricing of elec-

tricity based on the time of day when a consumer

uses electricity. Variable pricing is designed to

encourage customers to shift their heaviest use

from peak hours (8 am to 8 pm) to off-peak

hours (8 pm to 8 am). Variable pricing is also

based on the season. TOU rates are particularly

beneﬁcial to residential and small business cus-

tomers. Pricing is based on the marginal cost to

produce electricity. It depends on the total load

and generating units to serve that load. The

marginal cost of electricity production varies with

demand at a particular time of day. The primary

purpose of this rate system is to even out the use

of electricity to make demand more constant. It

is also designed to level the peak-and-trough

loads on electrical generation and strain of trans-

mission and distribution across a service grid, as

well as to decrease the cost for consumers who

decide to use electricity during off-peak times.

As most electrical utility companies are franchise

monopolies and their generating plants require

substantial capital construction costs, they want

to maximize the efﬁciency and generating

capacity of each plant before undertaking the

construction of additional plants.

Tolerance limits Environmental term designat-

ing maximum and minimum levels beyond

which a particular species cannot survive or is

unable to produce. There are tolerance limits in

temperatures, moisture levels, nutrient supply,

soil and water chemistry, and living space.

Toluene Soil contaminant and toxic chemical.

An organic liquid used in making benzene and

urethane. It is regulated through the US Safe

Drinking Water Act of 1974, which determines

safe levels of chemicals in drinking water.

Short-term exposure can result in minor nervous

system disorders, such as fatigue, nausea, weak-

ness, or confusion. Long-term exposure can pro-

duce nervous disorders such as spasms, tremors,

impairment of speech, hearing, vision, memory,

coordination, or liver or kidney damage. See also:

Soil contaminants; Toxic chemicals

Topping-cycle A method to increase the thermal

efﬁciency of a steam electric generating system

Topping-cycle 241

by increasing temperatures and interposing a

device, such as a gas turbine, between the heat

source and the conventional steam turbine gen-

erator to convert some of the additional heat

energy into electricity.

Total dissolved solids (TDS) Measure of the

total amount of all the materials dissolved in

water. These materials, both natural and anthro-

pogenic, are mainly inorganic solids with a

minor amount of organic material. The principal

application of TDS is in the study of water

quality for streams, rivers, and lakes, although

TDS is generally not considered a primary pol-

lutant. Primary sources for TDS in receiving

waters are agricultural runoff, leaching of soil

contamination, and point-source water pollution

discharge from industrial or sewage treatment

plants. The most common chemical constituents

are calcium, phosphates, nitrates, sodium,

potassium, and chloride, which are found in

nutrient runoff, general storm water runoff,

and runoff from snowy climates where road

de-icing salts are applied. TDS has a signiﬁcant

impact on the environment, not only because

of its contaminated pollutant content, but also

due to the effect the contaminants have on

water quality, plant and animal life, and con-

tinued healthy biodiversity. Reduction of human-

made wastes would mitigate the potential

adverse effects of TDS on the environment and

the ecosystem.

TOU See:

Time-of-use rate

Toxic chemicals USEPA lists the following as

toxic chemicals: benzene, chlorinated solvents,

chloroﬂuorocarbons (CFCs), dichloroethylene

(DCE), dioxins, endocrine disruptors, ether, ethyl-

benzene, furans, halons, hazardous air pollutants

(HAPs), heavy metals, hydrochloroﬂuorocarbons

(HCFCs), inorganic cyanides, ketones, methane,

methyl bromide, methyl chloride, methyl ter-

tiary butyl ether (MTBE), nitrogen oxides (NO

organic cyanides, particulate matter (PM), per-

chloroethylene (PCE), phthalates, polychlorinated

biphenyls (PCBs), radioactive substances, radio-

nuclides, styrene, sulfur hexa ﬂuoride (SF

), sulfur

oxides (SO

), toluene, trichloroethylene (TCE),

and volatile organic compounds (VOCs).

Toxic waste Waste material, often in chemi-

cal form, that can cause death or injury to living

creatures. It usually is the product of industry

or commerce, but also comes from residential

use, agriculture, the military, medical facilities,

radioactive sources, and light industries such as

dry cleaning establishments. As with many pol-

lution problems, toxic waste began to be a sig-

niﬁcant issue during the industrial revolution.

Toxins can be released into air, water, or land,

and can pollute the natural environment and

contaminate groundwater. See also:

Hazardous

waste

Toxics Also known as toxic substances. As

deﬁned by the US Clean Air Act Amendments

of 1990, toxics include benzene, 1.3 butadiene,

formaldehyde, acetaldehyde, and polycyclic

organic matter. Substances that are poisonous

and/or have an adverse affect on organisms,

organ systems, tissues, cells, or the environ-

ment. There are generally three types of toxic

entity: i) chemical—including lead, hydroﬂuoric

acid, chlorine gas, and organic compounds

such as methyl alcohol; ii) biological—includ-

ing bacteria and viruses; iii) physical—including

concussion, electromagnetic radiation, ionizing

radiation, direct blows, and vibration. See also:

Toxic chemicals

TPV See: Thermophotovoltaic cell

Trace gas Gases that make up less than 1% of

the Earth’s atmosphere: carbon dioxide, water

vapor, methane, and oxides of nitrogen, ozone,

and ammonia. Nitrogen, oxygen and argon make

up more than 99% of the Earth’s atmosphere.

242 Total dissolved solids (TDS)

Trace gases, although small in absolute volume in

the atmosphere, still affect weather and climate.

Tracking array See:

Photovoltaic tracking array

Tradable renewable certiﬁcates See: Green

certiﬁcates

Tragedy of the commons See: Open access

system

Transesteriﬁcation Chemical process that

reacts an alcohol with the triglycerides con-

tained in vegetable oils and animal fats to

produce biodiesel and glycerin.

Transpiration Process in which water is absor-

bed by plants, usually through the roots, and eva-

porated into the atmosphere from the plant surface,

usually the leaf pores. See also:

Evapotranspiration

Transpired air collector A type of solar col-

lector. It is made of dark, perforated metal. The

metal is heated by the Sun, and a fan pulls

ambient air through the holes in the metal,

which heats the air. Used for pre-heating venti-

lation and drying crops. Requires no glazing or

insulation and is inexpensive to manufacture.

Transpired air collectors are low-cost devices

and their potential is good in developing coun-

tries, where coffee, grains, fruit, vegetables, and

other crops are harvested, and conventional

fuels are expensive or unavailable.

Transuranic waste Category of radioactive

waste. Contains elements that have an atomic

number higher than uranium (92), such as plu-

tonium. Results primarily from past nuclear weap-

ons production and cleanup of nuclear weapons

facilities. There is concern about its safe disposal

and isolation.

Tribrid vehicle The third generation of alter-

native propulsion vehicles. The second generation

is the bivalent hybrid vehicles with turbines.

Tribrids are hybrids that obtain additional energy

from the ambient environment (solar panel,

windmill, or sail). Examples are a velomobile with

an electric power-assist motor and additional

on-board solar cells, like the new French pho-

tovoltaic car, the Venturi-Astrolab. See also:

Hybrid engine; Solar electric-powered vehicle

Trichloroethylene (TCE) Soil contaminant and

toxic chemical. Used to remove grease from

fabricated metal parts and some textiles. It is

regulated by the US Safe Drinking Water Act of

1974, which determines safe levels of chemi-

cals in drinking water. Ingestion increases risk

of cancer and liver problems. See also:

Soil

contaminants; Toxic chemicals

Trickle collector Type of solar thermal col-

lector in which a heat-transfer ﬂuid drips out of

header pipe at the top of the collector, runs

down the collector absorber and into a tray at

the bottom, where it drains to a storage tank.

Triple-pane window Three layers of glazing in

a window with an airspace between the middle

glass and the exterior and interior panes. This

construction provides more insulation and greater

energy efﬁciency.

Trombe wall A wall with high thermal mass

used to store solar energy passively in a solar

home. The wall absorbs solar energy and trans-

fers it to the space behind the wall by means of

radiation and by convection currents moving

through spaces under, in front of, and on top of

the wall. Trombe walls can provide carefully

controlled solar heat to a space without the use

of windows and direct sunlight. The masonry

wall is part of the building’s structural system,

effectively lowering costs. The inside, or dis-

charge, surface of the Trombe wall can be

painted white to enhance lighting efﬁciency

within the space (see FIgure 76).

Trombe wall 243

Trophic level Term for each level of energy

consumption within a food web. Energy within

a food web always ﬂows in only one direction,

starting with the producers. See:

Food Chain

Trophic status Classiﬁcation of a lake or body of

water. See also:

Eutrophic; Mesotrophic; Oligotrophic

Tropical rain forest Forests in the equatorial

regions of Central and South America, Central

Africa, and Southeast Asia. These forests have

ﬁve times more tree species than those in more

temperate regions. The environment is rich in

biomass and biodiversity.

Troposphere Lowest part of the atmosphere

from the surface to about 10 km (6 miles) in

altitude in mid-latitudes where clouds and

weather phenomena occur. In the troposphere,

temperatures generally decrease with height.