by Professor David Faiman
Ben-Gurion National Solar Energy Center,
Jacob
Blaustein Institute for Desert Research,
Ben-Gurion University of the
Negev
BACKGROUND
Israel is located at the geographic latitude of approximately 30
o N, where the annual incident solar irradiance is about 2000
kWh per sq.m. It has, however, no natural energy resources; all of the
country's electric power and fuel are derived from imported coal and oil.
At present, the electrical generating capacity of the country stands at
about 6.5 GW, representing approximately 1 kW per capita; but this value
has increased in recent years as the need for electricity, in all walks of
life, has risen. In such a situation it is, therefore, not surprising that
Israel has pioneered the use of solar energy. Furthermore, with large
areas of desert (approximately 60% of the country), it is natural that
extensive R&D should be underway in order to enable the Negev desert to
provide substantial amounts of solar-derived power in the future.
CURRENT USAGE OF SOLAR ENERGY
Domestic Hot Water
Perhaps the most common manifestation of putting the sun to work in Israel
are the solar water heaters that cover roof-tops all over the country.
Typical domestic units consist of a 150 liter insulated storage tank and a
2 sq.m. flat panel. The latter collects solar radiation, heats the water
and passes it to storage in a pumpless, gravity-driven loop. These systems
operate at an annual average efficiency of approximately 50%. It is
therefore easy to calculate that such a unit saves its owner some 2,000
kWh per year in electricity costs, raising the temperature of a tankful of
water by approximately 30oC above its starting point on an
average day - i.e. heating water to a temperature of about
50oC. This means that most days of the year there is no need to
employ the electrical backup heating coil (which all storage tanks
contain) in order to ensure that the water is warm enough for washing.
Larger systems, usually pump- driven, are to be found on high-rise housing
projects, on several kibbutzim and at a number of industrial plants around
the country.
Passive Solar Space Heating
Although Israel is commonly perceived as being a "hot" country winters can
be cold, particularly in Jerusalem and other highlands - including those
in the Negev desert. However, the climate is ideal for employing so-called
passive solar heating. Basically this means designing a house so that it
can heat itself with winter sunshine but remain cool in summer. The
alternative active form of solar space heating, which employs solar
collectors, electric circulation pumps and heat storage, although much
researched elsewhere in the world, is not cost-effective in Israel because
of the relative brevity of the winter season. The basic ingredients for a
passive solar house in Israel are: (1) A well-insulated building
envelope; (2) sufficient thermal mass to smooth out large temperature swings
and provide night storage; (3) an appropriate area of south-facing
windows. Typical solar houses in the colder parts of the country might
involve a wall structure comprising a 1 cm. thickness of plaster on the
inner surface, followed by 10 cm. of solid concrete for thermal storage,
then 5 cm. of polyurethane foam insulation and, finally, some finish,
depending upon local building codes, to protect the insulation. The roof
would include 10 cm. of polyurethane insulation and the total area of
south-facing windows would amount to about 15% of the floor area. In
warmer parts of the country proportionately smaller window areas are
needed. All windows would be provided with exterior blinds for reducing
the penetration of unwanted summer sunshine. The first passive solar house
in Israel was constructed from sun-dried adobe bricks and is located on
the Sde Boker campus of Ben-Gurion University. Since its completion in the
late 1970s the basic principles of passive solar design have been widely
adopted by architects all over the country.
Photovoltaic Rural Lighting
At the time of writing (1997) there is no manufacturing industry for
photovoltaic (PV) cells in Israel. This fact, coupled with the still
relatively high cost of PV cells, has resulted in a relative dearth of PV
demonstration projects despite the ideal climatic conditions the country
offers for this technology. One sector does exist, however, in which there
has been a relatively high penetration of PV into the public perception
and this is at rural bus stops. A number of private entrepreneurs import
the relevant components and market (usually to local authorities) lighting
units which comprise a PV panel, a storage battery, a low-power lamp and
control electronics for protecting the battery. In this manner, solar
power is used for lighting these bus stops during night hours.
INNOVATIVE SOLAR DEMONSTRATION PROJECTS
With the onset of the energy crisis of 1974 a number of innovative solar
demonstration projects were undertaken by Israeli industry and the
government. The two most prominent in the private sector were an
electricity- generating solar pond at the Dead Sea and a solar industrial
process heat system in the north-west Negev. In addition, the government
established a large solar test- demonstration facility in the Negev.
Electric Power from Saline Solar Ponds
The basic idea involves a pond of saline water, about 2 m. in depth, which
is artificially maintained so that the degree of its salinity (and
consequent density) is higher at the bottom than at the surface.
Absorption of solar radiation by the floor of the pond heats the lower
depths of water which are prevented from rising by their high density
relative to the upper layers. In such a situation the temperature of water
at the bottom of the pond continues to rise and is found to attain
temperatures close to 100oC. Furthermore, since the ponds are
very large - one demonstration pond, at Beit Ha'aravah, is 250,000 sq.m.
in area - this represents a huge amount of stored energy. The Ormat
Corporation who pioneered such ponds developed a special low-temperature
turbine which enables the hot pond water to convert an organic fluid to
vapor and thus produce electricity. For the Beit Ha'aravah pond, a 5 MW
turbine was built.
The thermodynamic efficiency of such a comparatively low-temperature
power-producing system is of needs small, approximately 1% at best.
Accordingly, one would expect such a pond to produce, on average, only
about 570 kW of electrical power. A 5 MW turbine would therefore, at first
glance, appear to be hopelessly optimistic. However, the unique feature of
solar ponds, compared with all other solar technologies, is their built-in
storage capacity. It takes several weeks until the pond temperature
achieves a steady state at its lower depths, after which, provided one
does not withdraw energy at an average rate that exceeds the nominal 570
kW on an annual basis, one can in fact achieve vastly greater power
outputs for a few hours each day - typically during the morning and
evening peak load periods. In effect one allows the pond to absorb solar
energy during the day but only operates the turbine in the early morning
and late afternoon hours. Ormat's organic fluid turbine has turned out to
have such a long life-time, partly because it is a totally sealed unit,
that such devices are to be found all over the world in situations where
low-temperature heat sources are available and electric power is
required.
Industrial Process Steam from Parabolic- Trough Solar Collectors
The second large, innovative, solar demonstration project was one
involving the use of parabolic-trough reflectors for producing industrial
process heat. This was a proof-of-concept project that Luz Corp. carried
out at a potato-chip factory in Sha'ar Ha'negev. For this purpose,
sun-tracking glass mirrors, curved so as to form long lines of reflecting
troughs, concentrated the sun's light onto a central tube through which
oil was pumped. The solar-heated oil, at temperatures in excess of
200o C, was then used to produce steam in order to provide for
the process-heat needs of the plant. Similar solar collectors were
subsequently employed by Luz in their much-celebrated, record-breaking,
12.5 MW electricity-generating power station at Dagget, California. With
this inroad into electric power production successfully accomplished, Luz
went on to construct six 30 MW power plants, employing a larger size solar
collector unit and even two 80 MW power stations which involved a third
generation of yet larger solar collector units. All of these
electricity-generating power plants were built in California. Although the
US plants are still fully operative, Luz went bankrupt before they were
able to complete negotiations for a similar solar power station in their
country of origin.
The Ben-Gurion National Solar Energy Center (BGNSEC)
In addition to investment in the two private-enterprise projects referred
to above, the government of Israel established, in 1985, a national solar
technologies test center at Sde Boker in the Negev desert. The original
purpose of the center was to demonstrate, in a comparative manner, the
various alternative solar technologies that appeared promising for
large-scale power production. These included a then-current Luz solar
oil-heating loop and a system of very large parabolic mirror troughs,
which were designed to heat water directly to steam rather than via an
intermediate oil-heating stage. Unfortunately, this system was never
completed by Luz and stands at Sde Boker, today, as a monument to a
commercial enterprise that came within a hair's-breadth of being able to
generate large-scale solar power in a truly cost-competitive manner. In
addition to the solar-thermal demonstration systems at Sde Boker a number
of photovoltaic systems were installed in a manner that would enable them
to feed into the electrical grid. In 1991 the government charged
Ben-Gurion University with the task of converting the BGNSEC into a solar
research facility. Today, under its new operators, research at the center
covers a wide spectrum of topics. In addition to electric power
production, photovoltaics are investigated both at the systems and the
device (new materials) level, solar radiation is studied both from the
energy and the environmental (UVB/ozone layer) viewpoints and a number of
large-scale projects, both at Sde Boker (giant parabolic dish) and in
other parts of the Negev (200 kW PV system at Kibbutz Samar), are at
various stages of planning and execution.
SOLAR R&D IN ISRAEL
Solar research and development is being carried out at a number of
universities and research institutes throughout the country.
The Negev Solar Radiation Survey was established by the Ministry of
National Infrastructure in the 1980s in cooperation with Ben-Gurion
University's BGNSEC and the Meteorological Service. The Survey documents
solar radiation (and other pertinent meteorological parameters) from
approximately 10 sites in the Negev, in order to identify appropriate
locations for solar power stations of the future and provide a data base
for their efficient design.
Photovoltaics, although having little if any industrial backing in Israel
at present, does enjoy a modest degree of government support because this
technology may form the basis of some of the power stations of the future.
Innovative methods for producing silicon solar cells are being
investigated at the Jerusalem College of Technology (high-efficiency,
single crystal cells) and at Tel Aviv University (amorphous silicon thin
layers). New thin-film materials are being investigated for potential PV
use at Ben-Gurion University of the Negev (C60), at the Technion Israel
Institute of Technology (CdTe) and at the Weizmann Institute of Science
(WSe2).
Solar-thermal power, another candidate technology for future power
stations, is under investigation at Ben-Gurion University (parabolic
troughs and a parabolic dish) and at the Weizmann Institute (solar furnace
and central receiver tower), the latter with the active participation of
industry. The Ben-Gurion University dish, to be located at the BGNSEC,
will be 400 sq.m. in area and capable of concentrating the sun's rays up
to 10,000 times. This is orders of magnitude higher than the concentration
available from linear reflectors such as parabolic troughs and will
accordingly permit a wide range of new research avenues to be
investigated. The Weizmann Institute Central Receiver Tower, on the other
hand, consists of a field of 64 so-called "heliostat" mirrors, each of
approximate area 50 sq.m. that re-direct the sun's rays to a boiler, or
some other suitable receiver, mounted on a tower some 50 m. in height. The
combined effect of so many mirror surfaces, when focused onto a relatively
small central receiver, can obviously produce extremely high solar
concentrations.
The Weizmann Institute tower should not, however, be confused with yet
another tower concept that is under active development at the Technion.
This idea involves pumping water to the top of a very high tower (1 km. or
more) which would be located in a dry desert area. The water would
evaporate and the down-draft created by falling, cooled, moist, air would
then drive a special wind-turbine located within the tower. This, of
course, is a secondary use of solar energy but one which, nevertheless,
has intriguing possibilities.
For further information:
Ben-Gurion National Solar Energy Center
Jacob Blaustein Institute for Desert Research
Ben-Gurion University of the Negev
Sde Boker Campus 84990
Tel: (972)-8-659-6934
Fax: (972)-8-659-6736