Lecture # 7 Solar Thermal Energy 1. Low Potential Heat презентация

Solar Thermal 1
Low Potential Heat
Greenhouses
Trombe walls
solar cookers
solar water heating systems
CPC collectors, troughs, vacuum pipes
implementation of solar ponds
Solar desiccant cooling
Integration with PV
Acquaintance with AUA solar hot water and desiccant cooling system

Solar thermal energy
With all objects on Earth, solar heating reaches a state of temperature homeostasis as the heat imparted by the sun is offset by the heat given off through reflection, radiation, and convection.
Visible
Visible
Infrared (IR)
IR
IR
Convection
Contact heat transfer

Greenhouse

Greenhouse

Greenhouse – controllable temperature

Solar thermal energy
White objects stay dramatically cooler than other objects because the most important variables are characteristics of the surface:
- reflectance, - emissivity,
- convection,
- surface area.
Slowly but surely an extinct glacier in a remote corner of the Peruvian Andes is being returned to its former colour, not by falling snow or regenerated ice sheets, but by whitewash.
Real snow on Chalon Sombrero - not paint
RESULT ?

Albedo = whiteness
The albedo of an object is a measure of how strongly it reflects light from light sources such as the Sun. It is therefore a more specific form of the reflectivity term.
Albedo is defined as the ratio of total-reflected to incident electromagnetic radiation. It is a unitless measure indicative of a surface's or body's diffuse reflectivity.

Various Albedo remember that relative value is important

Solar thermal energy
Silvery objects get hot even though they are excellent reflectors because they are very poor in heat emission. Human skin, and many other living surfaces, like tree leaves, have near perfect emissivity (~1.0), and so stay pretty cool.
Black people in Africa emit more IR than white people.
A perfect sunscreen is a dye that perfectly absorbs, with high emissivity, or perfectly reflects, ultraviolet and infrared while being transparent in visible light.

Made from metalized polyester film, this ultra-insulating material is coated on both sides with a heat-reflective metallic surface.
It is ideal for treating or preventing hypothermia after first aid has been administered. The emergency foil blanket retains most radiated body heat while protecting against wind and rain.
Foil Survival Blanket reflective thermal first aid

Solar thermal energy
Solar water heaters have solar collectors that are installed outside dwellings, typically on the roof or nearby.
Many models are the direct-gain type, consisting of flat panels in which water circulates.
Other types may use dish or trough mirrors to concentrate sunlight on a collector tube filled with water, brine or other heat transfer fluid. A storage tank is placed indoors or out.
Circulation is caused by natural convection or by a small electric pump. At night, or when insufficient sunlight is present, circulation through the panel can be stopped by closing a valve and/or stopping the circulating pump, to keep hot water in the storage tank from cooling.
Depending on the local climate, freeze protection, as well as prevention of overheating, must be addressed in their design, installation, and operation.

Passive Solar - Solar Cookers
A solar cooker is a way of using the sun's power to cook. There are mainly two types of solar cookers: concentrating and solar box cookers (usually known as solar ovens because of the way they are used).
Since they use no fuel they are free to run, humanitarian organizations are promoting their use worldwide to help slow deforestation and desertification caused by the need for firewood with which to cook.

Passive Solar: Solar Cookers

Passive Solar - Solar Cookers

Lusine Machyan, IESM 2009 – in Altoetting, Germany, learning solar cooker production technology.

Gegham Sargsyan, IESM 2009 – in Altoetting, Germany, learning solar cooker production technology.

Passive Solar - Trombe walls
A Trombe wall is a sun-facing wall built from material that can act as a thermal mass (such as stone, concrete, adobe or water tanks), combined with an air space, insulated glazing and vents to form a large solar thermal
collector.
Adobe wall ?
(not a Trombe wall)

Trombe Walls
During the day, sunlight would shine through the insulated glazing and warm the surface of the thermal mass. At night, heat would escape from the thermal mass, primarily to the outside. Because of the insulating glazing, the average temperature of the thermal mass can be significantly above the average outdoor temperature. If the glazing insulates well enough, and outdoor temperatures are not too low, the average temperature of the thermal mass will be significantly higher than room temperature, and heat will flow into the house interior.

Trombe Walls
Modern Trombe walls have vents added to the top and bottom of the air gap between the glazing and the thermal mass. Heated air flows via convection into the building interior. The vents have one-way flaps which prevent convection at night, thereby making heat flow strongly directional. This kind of design is an isolated passive thermal collector. By moving the heat away from the collection surface, it greatly reduces thermal losses at night and improves overall heat gain. Generally, the vents to the interior are closed in summer months when heat gain is not wanted.

Solar Architecture

Solar Architecture

Solar Architecture
Look at south (in northern hemisphere)
Use the sun ecliptics
Use Trombe walls
Use controllable windows
Use integrated PV
Provide Aesthetics

Solar water heating panels
A bucket of water can use solar energy too.
Need to prevent losses
Provide comfort of use through engineering
Provide best price per peak watt.

Tel Aviv Picture by Eric Arevshatyan, IESM’07

Tel Aviv

Tel Aviv

Solar Water Heating Panel:
flat-plate absorber, - intercepts and absorbs the solar energy,
transparent cover(s) - allows solar energy to pass through but reduces heat loss from the absorber,
heat-transport body - air, water or antifreeze, flowing through tubes to remove heat from the absorber
heat insulating backing

Solar thermal energy
With all objects on Earth, solar heating reaches a state of temperature homeostasis as the heat imparted by the sun is offset by the heat given off through reflection, radiation, and convection.
Visible
Visible
Infrared (IR)
IR
IR
Convection
Contact heat transfer

Solar Water Heating Panel
Low Iron Tempered Glass
Absorber
Selective coating
Rear insulation
Rear Screen (metal)
Piping, liquid
AIR

Selective Coatings
One of these tricks is "selective coating". In selective coating a thin layer of either finely divided nickel (black nickel) or chromium (black chrome) is formed on the surface usually by electrolysis.
Such a layer has the property that it absorbs solar energy almost as well as a matt black surface but it is a poor emitter of energy at the temperature of the base metal. The net result is that, other things being equal, a selectively coated absorber exposed to sunlight will get hotter than a simple matt black one.
This in turn means that the lower conductivity of an aluminum sheet can be compensated by having it run hotter so that the copper water tubes do not have to be put closer together and the sheet does not have to be made thicker.
A second advantage of selective coatings is that they enable the collector to work better in poor conditions (weak sunshine.

Efficiency:
vacuum pipe vs. flat panel

Efficiency
Conversion Factor: η0 = 0.717
Loss Coefficient: a1 = 1.52 W/(m2K)
Loss Coefficient: a2 = 0.0085 W/(m2K2)
G - insolation level in Watts/m2,
Ta - ambient temperatures
Tm - average manifold temperature (average panel temperature).
Tm = (Tinlet+Texit)/2

Main factors that influence SHW panel efficiency
Temperature difference between the inlet and outlet liquid, ΔT, depending on your working cycle.
Ability to isolate the convection through air.
Ability to block the infrared (IR) radiation.

Greenhouse

Vacuum almost totally prevent convection losses.

Evacuated pipe collector in CPC (curved parabolic) design

Vacuum collector

Solar Water Heating Systems
Passive – based on fluid convection
Active – based on forced circulation via pumps

Solar Water Heating Systems
Single Contour – no freezing environment, no need in antifreeze
Double or triple contour - freezing temperatures in the environment, need in antifreeze, certain hygiene norms.

SHW Systems

SHW Systems

SHW Systems

SHW Systems

Solar cooling
Photovoltaic + conventional air conditionaire.
Desiccant cooling.

AUA SOLAR SYSTEM

Desiccant Cooling
We have constant flow of inlet air, exhaust is dumped to the atmosphere.
Solar energy is used to regenerate the media that absorbs humidity.
Pre-desiccated room inlet air is humidified, low temperatures and acceptable humidity levels are attained.
The system is usually coupled with a heat-recovery system, that returns energy back to the room from the exhaust channel.

Desiccant Cooling

Desiccant Cooling

Desiccant Cooling

Energy factor and Solar Fraction
The solar energy factor is defined as the energy delivered by the system divided by the electrical or gas energy put into the system. The higher the number, the more energy efficient. Solar energy factors range from 1.0 to 11. Systems with solar energy factors of 2 or 3 are the most common.
Another solar water heater performance metric is the solar fraction. The solar fraction is the solar portion of the total conventional hot water heating load (delivered energy and tank standby losses). The higher the solar fraction, the greater the solar contribution to water heating, which reduces the energy required by the backup water heater. The solar fraction varies from 0 to 1.0. Typical solar factors are 0.5–0.75.

Heat pumps
A final development in water heating, which is not strictly solar but is sometimes set up in a "solar assisted" manner, is the heat-pump.
A heat pump is essentially a refrigeration system working in reverse. Instead of pumping heat from an evaporator inside the refrigerator to a condenser in the room the heat pump pumps heat from an evaporator in the atmosphere to a condenser attached to the hot water cylinder.
For convenience the evaporator is sometimes mounted on the roof of the house where the sun helps to provide input to the evaporator.
Despite the appearance of roof mounted heat pump evaporators, the heat pumps are really electrical heating systems which use electricity in a much more energy efficient way than simple conventional resistance heaters.
There is not a lot to distinguish between a good solar system and a heatpump in terms of overall net consumer savings over a year.

Windcatchers - Iran

Homework
Find from internet the total worldwide amount of solar water heating capacity installed. Copy paste the reference link.
Imagine your home has a south looking wall of a specific size (e.g. 3 x 6 m). Now you plan to build a Trombe wall on it. Roughly estimate the budget based on the bill of materials to construct it.