This paper presents a new design reference model for the development of economic, practical and technically feasible Space Based Solar Power (SBSP) systems that can provide a viable source of clean, renewable energy for markets on Earth.

The space based segment of the SBSP system described here can be constructed entirely on Earth and launched with currently available satellite launch vehicles.  No further on-orbit assembly would be required.   The Solar Power Satellites (SPS) described herein would operate in highly elliptical, 3 hour Sub-Molniya Orbits (rather than the Geosynchronous Orbits (GEO) of early SBSP designs) and utilize dish reflectors or Fresnal lenses to massively concentrate solar energy for both a High Concentration Photovoltaic (HCPV) power system and a Solar Dynamic (SD) electric power system.    
The SD system utilizes an Organic Rankine Cycle (ORC) thermal heat engine that, combined with the HCPV systems, will enable the conversion of 58% of incident solar energy for electric power production.

A Space Based Solar Power system of this design can provide a reliable, economically profitable and environmentally green, renewable source of energy for peak-power markets in high density regions of Western Europe, US, Canada, Japan and Russia.

The New SPS System Reference Design

Introduction

In order for SBSP concepts to become an economically viable source of clean energy, it is necessary to lower the SPS’s mass and size to the point that it can be launched into working orbit with currently available commercial launch vehicles – and without requiring any on-orbit astronaut assembly.  This is only possible by designing a smaller, yet more efficient, SPS system that would operate in an orbit closer to Earth.

This paper presents just such new design.  The SPS concept presented here is a hybrid Solar Dynamic (SD) and photovoltaic (PV) electric power generation and transmission satellite that would be placed in a Highly Elliptical Orbit (HEO).  An HEO is characterized by a relatively low-altitude perigee and an extremely high-altitude apogee.  Such an elongated orbit can have the advantage of long dwell times over the receiver during the approach to and descent from apogee.  A class of HEO proposed for use here is the ‘Sub-Molniya’ orbit.  A Molniya (Russian for “Lightening”) orbit is named after a series of Soviet/Russian communication satellites which have been using this type of orbit since the mid 1960s.  A satellite placed in a Molniya orbit spends most of its time over a designated area of the Earth as a result of “apogee dwell.”

Again, the solar power satellites described herein would be constructed on Earth, launched directly into orbit utilizing existing and available commercial satellite launch vehicles and would not require any on-orbit astronaut construction.  This concept negates the requirement for the development of a large astronaut construction force and new heavy-lift launch vehicles that have inhibited the economic and technical viability of previous SBSP designs. 

Outline:

SBSP system described herein consists of a constellation of 22 hybrid Solar Dynamic/High Concentration Photovoltaics Solar Power Satellites (SPS) and two ground power receiving stations.  The satellites are designed to combine the heat and power throughput generated by massively concentrating sunlight with Fresnal lenses or dish reflectors and convert this power to electricity for transmission to the Earth receiving stations via microwave.  The SPSs would be built on Earth and launched with currently available commercial launch vehicles into ‘Sub-Molniya’ orbits and would require no further assembly on-orbit.

The use of concentrating optics consisting of dish reflectors or Fresnel lenses are proposed to massively concentrate solar energy to intensities of 2000 suns or more onto the SPS’s photovoltaic cells.  Concentrating sunlight can significantly increase the energy exchange per surface area on photovoltaic panels and solar collectors compared with normal, incident sunlight.

In addition to increasing the power output of the photovoltaic cells, the concentrated sunlight would also generate a high level of thermal heat energy.  This thermal energy will be converted to electricity through the use of an active Organic Rankine Cycle (ORC) heat engine.  This hybrid system of thermal and photovoltaic electrical power ‘cogeneration,’ can convert approximately 58% of incident solar energy to electricity.

Description

The Sub-Molniya Orbit

The SBSP system herein described places the Solar Power Satellites in “Sub-Molniya” Highly Elliptical Orbits (HEO).  Sub-Molniya (Molniya; Russian for “Lightening”) is the name given to a class of highly elliptical orbits (HEO) with apogees of 3,800 km and perigees of 600 km. Satellites in elliptical orbits slow down near apogee and speed up near perigee.  Satellite orbits can thus be chosen to with apogee dwell or “hang time” of about 2 hours over the ground station receiver out of every 1 hour orbit.  This allows the satellite to transmit power to ground stations 16 hours out of a 24 hour period.

An example of the use of HEO Sirius Satellite Radio (Image 1) uses HEO orbits to keep two satellites positioned above North America while a third follow-on satellite quickly rounds the southern part of its 24-hour orbit.

High Concentration Photovoltaics (HCPV)

The High Concentration Photovoltaics (HCPV) systems of the SPS described here will employ concentrating optics consisting of dish reflectors or Fresnal lenses to concentrate sunlight to intensities of 2000 suns.  The increased concentration of sunlight results in higher efficiency and output of the solar cells.  In 2008, IBM demonstrated a prototype CPV to achieve an energy density of 2300 suns. Recently, Concentrix (Germany) and Amonix (USA) have announced operating AC efficiencies of 23% and 25%, respectively. These numbers point to significantly higher annual energy generation with HCPV than with competing technologies.

Organic Rankine Cycle heat engine PV cooling Solar Dynamic power system

In addition to boosting PV output, highly concentrated sunlight creates abundant thermal heat energy that would damage the HCPV system if it were not actively cooled.  This cooling is accomplished, with thermal heat being converted to electricity, by utilizing an Organic Rankine Cycle (ORC) thermal heat engine and Solar Dynamic power system.

To keep the HCPV solar cells cool and to utilize the thermal energy created by the concentrating optics, the ORC system would pump an organic coolant fluid via impinging jets and micro-channels through the photovoltaic array system.  In an ORC heat engine, an organic, high molecular mass fluid with a low liquid - vapor exchange temperature (boiling point) is used as a coolant.  As the liquid converts to vapor, work can be performed on a dynamic system which can then be converted to electricity.  In the case of the SPS, as the coolant heats up and vaporizes, it drives a turbo/alternator that generates electricity (the Solar Dynamic power system).  The vapor is subsequently channeled into a lower temperature reservoir where it condenses back into its liquid form to be recycled through the system in a continuous, closed system loop.

Several organic compounds have been used in ORC engines. Examples are: chlorofluorocarbons (CFCs), iso-pentane, toluene, ammonia and silicon oil.  Organic fluids can provide high turbine efficiency (up to 80%).  Another advantage of organic compounds is the fact that they do not need to be superheated, as with steam, and do not form liquid droplets upon expansion in the turbine which can cause erosion of the turbine blades.  Thus the use of organic fluids allows more design flexibility on the heat exchangers.

ORC technology is proven.  The efficiency of an ORC is estimated to be between 10 and 20%, depending on temperature levels of evaporator and condenser. An increase in evaporator- and/or decrease in condenser temperatures will give higher efficiencies. There are many examples of small scale ORCs that have been used commercially or as pilot plants in the last two decades. About 30 commercial ORC plants were built by 1984 with an output of 100 kW.  ORC heat engines have been shown to require little maintenance.  Their part-load performance is good.  Their start-stop procedures are simple and their operation can be fully automated.  As the ORC unit is a closed loop system, no replacement of coolant is expected to be required.

Again, an ORC heat engine Solar Dynamic power system is proposed to cool the Solar Power Satellites’ photovoltaic array while converting thermal energy to electricity.   It has been demonstrated that combining thermal and photovoltaic power generation can convert approximately 58% of incident solar energy to electricity.

The satellite uses Organic Rankine Cycle active photovoltaic cooling and thermal-electric conversion systems. The satellite has on-board electrical power storage using batteries. Power is transmitted to the ground via microwaves to two ground receiving stations, one in Japan and one in Western Europe. Each receives 1.1 GW of power per day for a total of 2.2 GW from 22 orbiting satellites. The satellites are small enough that they could be mass produced in factories similar to conventional communications satellites with no on-orbit assembly required.

Power Transmission

Each satellite will be designed to transmit 100 MW per orbit.  A constellation of 22 orbiting SPSs can thus provide 2.2 G MW of electrical power per day to two separate ground receiving stations.  Each satellite will have approximately 1.1 hours of beam time to the ground station and .4 hours away from the station. Due the long “hang time” of each satellite in a Sub-Molniya orbit, a total of 16 hours of transmission can take place out of a possible 24 hours, for a utilization rate of 66%.

CONCLUSION

The SBSP system described herein is a new baseline model that is technologically and economically feasible because the Solar Power Satellite segment of this system

• can be built on Earth in production facilities much like communications satellites are currently produced
• can be launched with currently available satellite launch vehicles
• would operate from 1.5 hour, Sub-Molniya, Highly Elliptical Orbits
• would not require further assembly on-orbit by astronauts
• would utilize concentrating optics to massively concentrate solar energy to boost photovoltaic throughput
• would also convert thermal energy to electricity via an Organic Rankine Cycle heat engine
• can be built, launched, operated and improved upon in a bootstrapping process with a short time-to-term for each satellite system.

With the small ground receiver array area required, this SBSP system is well suited to servicing high density regions.  An operational SBSP system of the design proposed herein can provide a financially lucrative and environmentally friendly, renewable electrical power resource to peak-power markets in Western Europe, US, Japan, Canada and Russia.