| Solar-thermal
systems |
| History |
| There
are currently two solar-thermal system variants: the parabolic-trough system,
on the one hand, in which the mirrors track the sun on a single axis,
from east to west, and the power tower, which operates
with heliostats that
track the sun on two axis, i.e., azimuth and east to west. Both systems
generate heat, which is used to produce steam to drive a turbogenerator.
The heat (thermal energy) can be stored in an absorber to permit
generation of electrical energy even in the absence of sunlight. |
| Trough
technology is intended for use in large installations of at least 50
MWe, linked to an energy transmission grid. The present-day
temperature limits of this system are around 400 degrees C. Maximum efficiency is up to 20 %. |
| Tower
technology is more flexible, since it does not need a circulation system
for heating of a fluid. The focused solar radiation
generates a temperature level of between 800 and 1000 degrees C at the
receiver on the top of the tower virtually immediately. This fact, and
the system's dual-axis tracking configuration, permit achievement of an
efficiency of 30 %. This system's energy output is determined by the number
and size of the heliostats. An additional advantage of tower technology is the generation of hydrogen
as a result of the high temperature level. |
| Problem |
| Both the
solar-thermal systems described above have the advantage that they generate
electrical energy without any fuel such as coal, gas or uranium, and
thus produce zero emissions. Their disadvantage is the high level of
investment necessary compared to conventional plants. It has, up to now,
not been possible to achieve a commercially acceptable kWh price for
generation using solar-thermal systems. |
| The most
important study of the reduction of heliostat costs was drafted for the
US Government by Sandia National Laboratories (Sandia Report 3293, 2007).
This report examines only electrically driven heliostats, a number of
generations of which have been developed in the USA since 1975. The report
concluded that the pedestal-mounted heliostat with electrical drives
for elevation and azimuth-rotation should be selected as the basis for
further development. Unfortunately, the azimuth-drive system is the most
expensive component, accounting for one third of total costs, as a result
of the fact that a step-down gearing system of approx. 1:33000 from the
electric motor's output speed is necessary to permit tracking of the
sun. The assessment board thus concluded that expansion of the surface
area of each heliostat mirror to 150 m2 would be the best method of reducing
costs. Even an annual production of 50,000 heliostats (equating to a
generation capacity of 600 MW/a!) would still not permit achievement
of the target of a substantial price-reduction, however. |
| The use
of a hydraulic drive system was also investigated, but was rejected due
to the excessive complexity and maintenance costs involved. It is our
assumption that the intention was to use an electronic servo-system to
achieve the necessary precision. Such a system would, indeed, be extremely
complex and costly. |
| Task |
| We will
examine below only the power tower system and, in particular, the heliostats
used in it. The heliostats are the dominant cost element in a power tower
installation, typically making up over 50 % of total installation costs.
The tower itself, a single one-piece component, offers fewer potentials
for any significant cost reduction. The many hundreds of heliostats required
for a solar-thermal plant also need redesign to achieve a low-cost type. |
| Solution |
| The simplest
solution to achieve a linear motion and limited rotation is the use of
double-acting hydraulic cylinders. This is a well
known technology, proven in literally billions of applications. A hydraulic
cylinder (hydraulic ram) supplies powerful forces to withstand wind loads
and has no tolerances when in its retracted position, which is not the
case with a geared drive system. Hydraulic energy can be stored in an accumulator,
also making it possible to move the mirrors during power failures. This
is a great advantage, since failure of the energy supply is a not infrequent
occurrence during stormy periods. It is necessary, if a storm
is approaching, to set the mirrors to their horizontal position within
a short time.
A power failure during sunny periods will result in malfunctioning of the
heat-pump system in the tower, with the consequence that, only a few minutes
later, the receiver mounted at the top of the tower will be seriously damaged
by the rapidly rising temperature. It is imperative in such cases to divert
the beam away from the receiver in a matter of seconds, to avoid damage. All this is made possible by the hydraulic accumulator, which contains
sufficient energy to move the cylinders to the necessary predetermined
positions. |
| The hydraulic
valves are controlled and actuated using a small energy supply from a
DC battery. A further advantage of this system is its extremely
low internal energy consumption, the result of the fact that the hydraulic
pump runs only for
a few minutes each day, in order to charge the accumulator with hydraulic
fluid. The microvalves in the hydraulic control system each operate on
10 W, with switching times of 20 msec. The costs of a heliostat field
can also be decreased by using a separate power unit for individual sections
of the field, or for the whole field,
depending on its size. Each individual heliostat then only contains the electrical control equipment
and the valve block. |
| We have developed a very simple solution, involving four
microvalves and extremely low-velocity movement for stabilization at the
extremely narrow target position by means of repeated digital steps in
each direction. We have also developed for this application a small computer
unit which interchanges data with the field computer. |
| Where
heliostats are used in high-insolation (incident solar radiation, "high-sunshine")
regions of the earth, it is our opinion that the target should be that
of developing a standardized, medium-sized
heliostat. A greater or lesser number of installed heliostats, in combination
with appropriate selection of tower dimension, could well provide the necessary
self-sufficient energy solution for a large range of widely distributed
locations around the globe. |
| Why a
medium-sized heliostat? |
| The
mirror areas of the prototypes constructed up to now vary in surface
area from 8 m² to 150 m², realized in several countries: Australia,
Israel, Spain and USA.
Small heliostats with simple electrical linear drive systems are relatively
low-cost, and achieve good beam concentration. They necessitate a large number
of individual foundations and numerous electrical control units and wiring,
connections, etc., however. Larger heliostats require fewer foundations and
electrical equipment, but are expensive. In addition, their structure is
heavier and of a more complicated design, due to the need to withstand higher
wind loads. |
| There
are, essentially, four reasons for the development of a 30 m² heliostat: |
| 1. |
Wind
load
The German DIN 1055 standard, Part 4, provides data for calculation of
structures exposed to wind loads and, in particular,
to wind gusts, the energy contents of which differ with the diverse topographies
encountered in the various regions of the earth. European development
is aiming initially for installation in Mediterranean and Middle Eastern
countries. Sandstorms on flat terrain here can reach
180 km/h. The DIN standard classifies this as Category 2 on the Saffir-Simpson
scale; in this case, structures of less than 10 m above ground level are
relatively stable and safe, especially when designed to have a small wind-attack
cross-section.
|
| 2. |
Transportation
All
the components of the heliostat must be designed in such a way that
they can be transported by road in standard containers and assembled
by semi-skilled persons using unsophisticated tools and erected using
simple lifting equipment.
|
| 3. |
Mirrors
The
incorporation of larger numbers of mirrors into a single heliostat
necessitates many adjustments
and occasional corrective maintenance.
The four mirrors are automatically inclined toward
and secured at the center-point, dependent on focus distance, in order
to focus the beam on the tower. Four large mirrors simplify heliostat
structure,
transportation
and on-site
assembly.
|
| 4. |
Tower
size
Smaller mirrors focus the beam on the receiver better,
permitting smaller receiver surface
areas, less weight, and therefore lower-cost towers. |
The
HydroHelio™
|
| Our
heliostat is marketed under the "HydroHelio" trademark. |
30m² mirror area
|
 
Elevation betwen 0-90 degree
|
Stow position
|
Hydraulic power unit
|
| Technical
data: |
| 1. |
Overall geometry |
|
| |
Height |
5500
mm |
| |
Width |
6100
mm |
| |
Facets |
4 |
| |
Facet dimensions |
3000x2500x3
mm |
| |
Mirror area |
30
m² |
| |
Elevation point EL |
2950
mm |
| |
Azimuth point AZ |
2770
mm |
| |
Weight |
|
| |
Steel
structure |
375 kg |
| |
Mirrors |
210 kg |
| |
Hydraulic drive |
45 kg |
| |
Total |
630 kg |
| 2. |
Optical properties |
|
| |
Clean reflectivity (PMAM) 1.5) |
93 % |
| |
Focal length |
100 m |
| |
Beam spread, at 100m |
2 mrad |
| |
Positioning error |
0.5 mrad = 1/36 degree |
| 3. |
Drive |
|
| |
Type |
Hydraulic
system, two cylinders, dia. 50 mm for AZ and EL, hydraulic power unit,
electrical |
| |
|
motor, pump, tank, accumulator and four microvalves. |
| |
Hydraulic fluid |
Vegetable oil |
| |
Parasitic consumption |
70Wh/8 hours |
| |
Power supply |
0.25 kW |
| |
Positioning speed |
1
mm/s |
| |
Movement angles |
AZ
+/- 90 degree/ 8 hours (+/- 180 degree on demand) |
| |
|
EL
70 degree/ 8 hours
|
| |
Stowing speed |
0 to 90 degree within max. 10 min
|
| |
Wind velocity |
Working limit = 15 m/sec (54 km/h) /Survival limit = 50 m/sec
(180 km/h) |
| |
|
|
| Comparison of ATS (Advanced Thermal System, USA) against
HydroHelio™ |
| Mirror area |
m² |
148 |
|
30
|
|
| Total weight |
kg |
6385 |
43
kg/m² |
630
|
21
kg/m²
|
| Steel structure |
kg |
4006 |
27
kg/m² |
375
|
12,5
kg/m²
|
| Mirror |
kg |
1518 |
10,3 kg/m² |
210
|
7
kg/m²
|
| Drive EL+AZ |
kg |
816 |
5,5
kg/m² |
45
|
1,5
kg/m²
|
| Positioning error |
mrad |
2 |
|
0.5
|
|
| * Thickness: ATS 4 mm, HydroHelio 3 mm |
|
| |
|
|
| High
Concentration Photovoltaics |
|
HCPV technology, developed from conventional photovoltaics,
which employs silicon cells of lower efficiency, however, has recently
been developed to commercial maturity. In the new system, Fresnel lenses
are used to concentrate the incident sunlight on to special chips of
a square centimeter in size, generating an electrical current. This
technology achieves a further significant increase in efficiency, to
35 %. It requires the tracking and positioning accuracy of a heliostat,
however, since obliquely incident light cannot be concentrated on to
the chip.
|
The hydraulically actuated
HydroHelio™ is particularly suitable
for use as a tracking system in combination with HCPV. Its high positioning
accuracy and high, backlash-free position-holding forces make it possible
to achieve optimum performance figures in the conversion of solar radiation.
The combination of 30 m2 HCPV and the HydroHelio™ tracker permits
generation of 6 KWe of AC electricity.
|
LEHLE GmbH
Technik & Design
robert.lehle@lehle-gmbh.de
fax: +49 (0) 7034 26661
Teckstrasse 37
D-71116 Gärtringen
Germany
Registered: HRB 3902
Amtsgericht Böblingen
VAT-Nr.: DE811926159 |