THE SOLAR POWER SATELLITE

                                                    Abstract:

The selling price of electrical power varies with time. The economic viability of space solar power is maximum if the power can be sold at peak power rates instead of baseline rate. Demand varies roughly by a factor of two between the early-morning minimum demand, and the afternoon maximum; both the amount of peak power, and the location of the peak, depends significantly on the location and the weather.  New designs for a space solar power (SSP) system were analyzed to provide electrical power to Earth for economically competitive rates. The approach was to look at innovative power architectures to more practical approaches to space solar power. A significant barrier is the initial investment required before the first power is returned. Three new concepts for solar power satellites were invented and analyzed. The integral-array satellite had several advantages, including an initial investment cost approximately eight times lower than the conventional design.

Introduction:

The Solar Power Satellite is a concept to collect solar power in space, and then transport it to the surface of the Earth by microwave (or possibly laser) beam, where it is converted into electrical power for terrestrial use. The recent prominence of possible climate change due to the “greenhouse effect” from burning of fossil fuels has again brought alternative energy sources to public attention, and the time is certainly appropriate to reexamine the economics of SPS. In the analysis of the economics of solar power satellites to provide electric power for terrestrial use, past analyses have typically assumed an averaged (or "baseline") power pricing structure. In the real world, price varies with location, season, and time of day; and the initial markets for satellite solar electricity need to be selected to maximize revenue. It is important to design the system to service the real-world electrical power market, not to an unreal average-price model. The following criteria will have to be used for a credible analysis of solar power satellite economic benefits and rate of return:

Satellite power generation should fit electrical demand profile

Satellite power generation should generate power at the maximum selling price

Use actual data on electrical demand & price

         

 Earth-Sun L2 Design details-:

                                                        

The space power system designed to be located Earth-Sun L2 will be radically different from conventional GEO Space power concept Since the sun and Earth are nearly the same direction, it can feature:

1.Integrated solar concentrator dish/microwave transmission dish

2.Integrated solar cell/solid state transmitters

3.No rotating parts or slip-rings

Frequency: 30 GHz: efficiency is lower than 2.45 GHz, but much tighter beam

4.transmitter diameter: 3 km

5.receiver diameter: 6 km

6.3 ground sites, receive 8 hours per day 33,000 16.5 meter integrated PV concentrator/transmitter elements

7.Concentrator PV efficiency 35%

Based on L'Garde designs for inflatable microwave antennas, it should be possible to make a 16.5-meter concentrator/antenna dish for 15 kg. The solar array/solid state power amplifier array adds an additional 9 kg, for a mass of 24 kg per element .

                                                        

                                                           

Table: Super-synchronous Solar Power Satellite: Mass and power

Mass of mirror element

Similar to the design flown on the shuttle

Solar concentration ratio 50, focal plane area 4.28 square meters

Focal plane array mass is 9 kg

total mass per dish is 24 kg

PV power per dish is 100 kW

Total Mass

Inflatable PV concentrator/transmitter elements mass 15 kg each (L’Garde design) PV mass 9 kg each (50x concentration) Structural mass 500,000 kg

Total Mass 1,300 tonnes

At assumed transmitter efficiency 33% (today’s technology): 1 GW power output

At assumed transmitter efficiency 67% (future technology): 2 GW power output

Disadvantages of Earth-sun L2 Solar Power Satellite

1. Size. The satellite-Earth distance of 1.5 million km means that the physics of diffraction demands a large size. This means that the initial cost will be high.

2. Electrical generation profile. The design produces power primarily during the night. For the existing U.S. power market, the maximum power usage is during the day.

Fixed Geo synchronous Solar Power Satellite

One aspect of the design remains extremely attractive: the absence of a rotary joint makes the L2 solar power satellite a design with no moving parts in geo synchronous orbit. The baseline figure of merit for this design was to examine how the power production profile fits with the demand (and price) profile for terrestrial electrical power, assuming that the power is to "fill in" for a ground solar power system. The satellite designed with the same design criteria: maximum simplicity; no moving parts; mission is to power when ground solar power is not available.A fixed microwave transmitter is permanently mounted on a bifacial solar array, which can be illuminated from either side. The maximum power dawn and at dusk, with zero power production at noon and at midnight . This fills in for a hypothetical solar array on the ground, which produces maximum power at noon and zero power at dawn and dusk. By employing a fixed transmitter attached to the solar array, the power management and distribution system size can be greatly simplified and reduced in mass. The difficulties associated with power transfer from the array to the transmitter are minimized, and the mass and cost of the SPS are reduced. The new SPS needs only gravity-gradient stabilization to ensure that the transmitter remains pointed to the rectenna site on the Earth. The solar array is now a simple flat structure to support the photovoltaic solar cells. Since the array is designed to have cosine illumination, a complicated structure is not required to point the arrays to the Sun. Therefore further mass and cost savings may be realized. Over the course of a day, the fixed array produces 64% of the energy of a tracking array of the same size.

Analysis Using "Space Segment Model" Spreadsheet

 The purpose of the Space Segment Model is to evaluate the impact of technology and design choices on the mass, performance, and cost of various solar power satellite (SPS) concepts using a common model.

                                                                

Figure. Solar power satellite design with fixed                  Figure. GEO solar power satellite provides microwave transmitter (no moving parts).               maximum power at 6 AM and 6 PM.

In the Input worksheet, the user chooses various SPS concepts, architectures, and orbital parameters. The chosen parameters are used in the various other worksheets according to their purpose, and the relevant values are output to the summary worksheet. The user may go to each specific worksheet to examine how performance and cost characteristics are evaluated, and may make changes to these worksheets. This allows the user to customize to some degree the SSM to fit the SPS concept understudy. For the purposes of this study, the SSM provides more analysis than required. Several of the worksheet calculations were not applicable, such as market cities and interplanetary trajectory calculations. For the applicable worksheets, namely Solar Collection, Power Management and Distribution (PMAD), Power Transmission, Structure, and Propulsion, default values were used in many cases, as they were appropriate to the concept under study. When necessary, relevant values in the applicable worksheets were altered to suit the concept under study. By selectively altering the SSM, the model was used to determine the viability of the concept under study.

                                                                       

Figure. The Space Segment Model was used to perform a first-order sizing of the concept. By inputting the desired values of SPS concept, structure type, orbit type, power delivered, photovoltaic cell type, transmitter frequency, etc results of system and subsystem mass and cost were output to the Summary worksheet. Then relevant values in the appropriate sub-system worksheets were altered to better reflect the proposed design. A standard design was compared with the fixed design. The design considered is a bi-facial solar array which would require two arrays with power scaled to deliver 1 GW ground power scaled accordingly with values from the Solar Conversion worksheet. Thin film solar cell arrays were assumed.  Transmission frequency of 5.8 GHz was chosen. Higher frequencies suffer from unacceptable atmospheric attenuation, and lower frequencies require larger transmitter arrays and/or rectennas. When revising the Space Segment Model to fit with the proposed concepts, each subsystem can be modified to a certain degree, and some cannot be modified at all.

 The differences between the baseline and the fixed GEO systems are: Transmitter subsystem: no modification since it is identical in both cases. It is sized according to power output and frequency; other SPS variables have no effect on the transmitter. Solar conversion subsystem: revised to be a bi-facial array, as opposed to a single sun-tracking array. Attitude control and orbit maintenance: reduced since the fixed SPS would be gravity-gradient stabilized. While station keeping cannot be neglected in a thorough design, for a first-order sizing it may be assumed to be negligible in terms of overall system mass and cost. Robotic subsystem: responsible for the construction of the SPS in LEO, and so is not affected to a large degree by the simplified system. Structure subsystem: a simple structure was chosen in the Input worksheet, and thus mass or cost reductions cannot be realized directly. However, it is assumed that both solar arrays can be fixed to the one structure, so mass and cost savings are indirectly realized. Telecommunications subsystem: ignored; negligible mass and cost as fraction of total SPS. PMAD subsystem: The mass and cost of the cabling are eliminated, since very little cabling is required. Also, since the transmitter is fixed to the solar array, there is no need for a rotary joint. However, a sizeable portion of the PMAD subsystem is attributed to the voltage converters, which are necessary to transfer GW order levels of power to the transmitter.

 Thermal subsystem: incorporated throughout the SPS, and is evaluated in conjunction which the other subsystems. Propulsion subsystem: required to move the SPS from LEO to GEO, is dependent solely upon the overall SPS mass, and so is automatically calculated.

Integration and testing: automatically evaluated according to the other subsystems. Listed below is a summary of relevant output values, followed by the revised values generated by modifying the relevant values in the appropriate sub-system worksheets. In table it is assumed that a bifacial solar array can be produced at no additional cost or weight. Compared to the baseline, total mass savings is 3%, but total cost reduction is nearly 10%. The cost associated with an additional solar array is substantially greater than the mass and cost savings realized through satellite design simplification. It was anticipated that cost reductions from a simplified power management and distribution (PMAD) system would be large, however, the PMAD system accounts for only 2% of the overall system cost, regardless of it being a substantial portion of the system mass. Therefore PMAD cost savings do not have a large effect on the overall system cost, and consequently is unable to offset the increased costs from the additional solar array. For the proposed concept to be lower in cost than the baseline, it is necessary that the cost per watt of the solar cells be reduced significantly. For example, a 50% reduction in thin film photovoltaic cell cost, from $1/watt to $0.50/watt [9], would result in a system cost equal to that of a single-array SPS. In other words, the previously calculated savings of 3% mass reduction and 10% cost reduction would be possible. The proposed SPS design could then become economically feasible (at least according to a first-order calculation). Examination of the power price profiles for candidate urban areas, indicated that the cosine power production peaking at sunrise and sunset did not well match near-term power demand. Even if a noon peaked solar generation is subtracted from the demand curve, the power profile still does not perfectly match requirements. Much of the power is produced when the power demand is very low (e.g., before 8 AM), and electricity price is low. Since the power profile of the proposed design is not suited for selling at peak demand, and much of the power produced will not be sold at peak price, the higher energy cost per giga-watt-hour means that the design is not economically feasible in the near-term compared to the base-line.

Fixed Design with integrated microwave transmitter: the "8 AM/4 PM" design.

If the design constraint of a single array is relaxed, two arrays can be baselined, and the arrays can be tilted outward to accommodate the actual demand peak (after subtraction of solar) at 8 AM and 4 PM (or other times chosen to fit the peak demand). With the addition of tilt, it is no longer true that the microwave beam is perpendicular to the solar arrays. The backside of each solar array is in the view of the Earth. A significant difficulty of the earlier design is the fact that the initial size of the system requires an extremely high initial investment. Due to the risk of the investment (market risk as well as technical), such investment is unlikely to occur. The redesign of the solar power satellite opens the possibility of integrating the solar array directly to the  micro-wave transmission [5,6]. By placing solid-state microwave transmitters directly on the back of the solar array, power management and distribution, as well as all voltage conversion, is eliminated. Figure 9 shows the conceptual design for a satellite to deliver maximum power at 8 AM and 4 PM, where the back  side of each array is an integrated microwave transmitter. This design was presented at the third SERT technical integration meeting [13]. The advantages of integration of the solar arrays and the transmitter are discussed in reference [5] and [6]. By integrating solar array with the microwave transmitter, the transmitter aperture becomes as large as the solar array area. This results in a narrower beam. A narrow beam allows smaller rectenna areas, thereby permitting much smaller solar power satellites. The smaller scale reduces the initial capital investment. For "conventional" SPS designs, the ratio of solar array area to the transmitter array area is approximately a factor of 64.

                                                                

                                   Figure: notional design for a solar power satellite to deliver peak power at 9 AM and 4 PM.

Very large scale integration Each solar-array element incorporates microwave transistor on  reverse side Reverse side of solar array acts as phased array antenna (Phase signal must be distributed to each element) For the integrated design, the transmitter area equals the solar array area. For the same power density on the ground, the minimum system size decreases in power by a factor of 4 for the 4 PM/8 AM tilt design. The rectenna area scales proportionately, and the minimum investment cost to first power decreases. Overall, an integrated 4 PM/8 AM fixed delivers same peak power, but 2/π (64%) lower total energy than a fully tracking SPS. The power is delivered at peak-power rates, not baseline power rates, resulting in two times higher revenue per kW-hr. Thus, the integrated SPS delivers 27% more revenue at 30% lower cost.                The bottom line is that the integrated SPS delivers power at 45% lower cost.

By reducing the size of the SPS to take advantage of the narrower beam, an integrated SPS can be decreased in power by factor of 4. This means that the cost to first power can be reduced by factor of 5.7. Since the investment required to reach first return is the major showstopper for the economic case for space solar power, this is a significant improvement in the design.

Fixed Design with integrated microwave transmitter: the "Slab" One-sided array

The 8 AM/4 PM design has two leaves in a "dihedral" configuration. It is evident, however, that the operation of the two leaves are independent of each other. This brings up the possibility of making a solar power satellite with just a single leaf: a "slab" design, with the solar energy incident on one side and the power beamed out the other.  For the same power density on the ground, the minimum system size decreases in power by a factor of 8 for a face-on solar array. This is even better than the factor of 4 found for the 4 PM/8 AM tilt. The tilt of the system can be chosen to provide power that is optimally adjusted to the peak power requirements. For example, a tilt of 30 degrees could be used to provide peak power at 2 PM. This matches the maximum power demand of urban areas in the United States. This peak can be adjusted forward or back, subject to the constraint that peaks at (or near) 6 AM and 6 PM are not possible, since these would require the array to be edge-on to the direction of microwave beam. Figure 10 shows a slab" one-sided array tilted to produce peak power at 2 PM.

                                                                      

 Figure. "Slab" single-sheet solar array, tilted to provide peak power at 2 PM.

In this design, the 2 PM tilt is not a gravity-gradient equilibrium. Maintaining the tilt will require

stabilization. For example, a gravity-gradient boom could be deployed downward on a truss to put thesystem into gravity-gradient stability.An alternate version would be to orient the solar array horizontally, and to direct the beam at an angle,to reach a receiver located at a slightly eastward latitude. The horizontal orientation is an equilibrium inthe gravitational field, but weakly unstable (this is the orientation of the ISS, for example). By aiming thepower beam approximately 2500 km east, the “noon” power beak generation for the satellite can be received at2PM. 

                                                                  

 Table .shows a space-segment model of the "slab" design. The cost of the design is 64% of the cost of the conventional tracking array. The conclusions of this analysis are:

1.Lower total energy, but power is matched to peak demand

2.At the same size, system delivers 64% of the power at 64% of the cost

3.But power sells at 2 PM peak power rates, not baseline power rates

4.minimum size can smaller by factor of 8

5.Eight times lower investment to first power

6.8 times more attractive

Power Transmission System:

Power transmission from the satellite is made by 2.45 GHz micro-wave beam emitted from the spacetenna, the antenna onboard the satellite. The beaming angle as large as 60 degrees of this case makes this requirement more important than in the case of the Reference System.

 

             Table: Spacetenna Characteristics

                   Electrical Characteristics

Frequency	2.45GHz
Beam control	Retrodirective
Beam scanning angle	+30 degrees (east-west)
	+16.7 degrees (north-south)
Power distribution	constant
Power density	574W/m2
Max. power density on ground	0.9mW/cm2
Input power to spacetenna	16 MW
Transmitting power	10 MW

                                           Mechanical Characteristics :

Shape and Dimension	132m x 132m square
Mass	134.4 ton
Number of Array module	88
Number of subarray	1936
Number of antenna elements	2,547,776 units
Number of pilot receiver	7,744 units

 Conclusions:

A space solar power generation system can be designed to work in synergy with ground solar power. Previous Space Solar Power architectures were designed to deliver 24-hour power; this design constraint was relaxed. A non-tracking, integrated solar/microwave Space Power system can be configured to match peak power demand .The economic case for a solar power satellite is most compelling if the solar power satellite can generate power that sells at peak, rather than average, price. Data from New York and Boston were examined to determine when the peak power prices occur. Several new designs for solar power satellites were considered, in an attempt to maximize the amount of power produced at peak rates.