Economic Value Added By

ECONOMIC VALUE ADDED BY

SEAWATER DESALINATION TECHNOLOGY TO GROUND AND SPACE BASED SOLAR POWER SYSTEMS

Gopalaswami R1 and Kumaravel M2

[1Former Chairman & Managing Director, Bharat Dynamics, Hyderabad, India

2 Former Head, Central Electronics Centre, Indian Institute of Technology Madras, India)

March 2013

Keywords: reverse osmosis, terrestrial solar power, space solar power.

Synopsis

Water is a compelling necessity for mankind and India among other nations faces a serious water crisis compounded by an energy crisis The value added (in financial terms) by sale of fresh water produced by a terrestrial solar power based seawater desalination plant is over 45 times higher that by direct sale of solar electric power. Thus selling electricity from solar powerplants in India is currently not commercially viable and needs large State subsidies; however selling fresh water generated by solar electricity is not only a commercially viable but potentially a very profitable proposition. A square meter of solar array based in outer space generates 20 times more electricity than when located on ground. Hence seawater desalination using space based solar power and sale of fresh water instead of solar electric power is a most attractive proposition because the higher capital utilization of solar panels in space; and the value added by sale of fresh water by seawater desalination would effectively cover the additional high costs of space infrastructure (space transportation, space assembly and power beaming from space to ground)

Introduction

The severe scarcity of surface water resources and rapidly falling underground water levels accentuates the need for additional water supplies in many arid and semi-arid regions of the world. Like fossil fuel resources, water scarcity is likely to be considered as one of the major survival issues for many countries. Looking at the different sources of water shown in Table-1,

Table 1

Water Source / Availability
Ocean / 97.2%
Polar Ice Caps / 2.1%
Fresh Water on Earth / 0.6%
Brackish Water / 0.10%

One can see that seawater that provides 97.2% of all available water on planet earth is the only reliable source which could provide perennial supply of fresh water through desalination process. Hence, there is a compelling need to create new sources of drinking water from the oceans for population centers and industries in coastal regions,as a supplement to existing fast depleting water resources in the land. One way is by use of seawater desalination using Reverse Osmosis technology

Reverse Osmosis (RO): Working Principle Almost half of the desalination plants installed in the world use RO process. A typical SPV-SWRO plant process layout is shown in Figure 1. In the SWRO system, the pretreated and filtered seawater is pumped at high pressure (typically 70 bars) into several chambers containing membrane modules to get two different streams i.e. fresh water and brine at its output.

The pressure drop across the membrane unit is about 1.5 to 2.0 bars. Just about one decade ago, the brine from the membrane modules at high pressure was discharged into the ocean without recovery of pressure energy from the brine discharge. Recently, pressure recovery devices have been invented and patented where a large amount of this waste energy is used to boost the pressure of seawater feed into the high pressure RO pumps thus greatly increasing the process efficiency.

An important specification of an RO plant is its Specific Desalination Energy (Electrical, (termed εdsh), the energy that is required to get freshwater of one cu.m from the output of the plant. In 1970s this was about 15-20 kWh; whereas today it is down to les than 2.0 kWh/ cu.m of fresh water output. This was enabled by the recent innovation of the Energy Recovery Device (Pressure Recovery Device known as Pressure Exchanger). It is this Pressure Exchanger (from brine outlet) technology innovation that has made seawater desalination an attractive proposition by using solar energy

However, the process of desalination requires large quantities of electrical energy Environmental pollution caused by burning fossil fuels for desalination is a major concern. Hence, the harvesting of the solar energy falling perennially on wastelands in any country for generating electricity, or along the coast line to produce fresh water by seawater desalination are seen as promising options; provided the cost of electricity from solar power plants is competitive with that from thermal/nuclear/wind and other renewable energy based power plants.

Cost of Terrestrial Solar Power and Electricity The global trend in cost of solar power from 1991 to 2011 is illustrated in Figure 2. The cost of terrestrial solar power (TSP) in India is currently $1.6 million/MW. At these current costs of solar power in India, the cost of solar electricity from terrestrial solar powerplants SPV is Rs 6.48 /kWh

Economic Value Added to Terrestrial Solar Photovoltaic Energy by Integration with SWRO System A typical market segmentation in India, consists of rural (60%)-domestic (30%)–industrial (10%) for which the average selling price of fresh water in Southern India is currently (2011) Rs 1.60 per litre. Every one kWh of solar electricity generates (typically) about 182 litres of fresh water[1]. A typical SPV-SWRO plant selling fresh water will thus generate (182x1.60) i.e. Rs 291 per day with a SPV plant generated electricity cost of Rs 6.48 /kWh.

Hence the economic value added to solar energy by the SWRO process is an amplification factor by 291/6.48, or about 45 times. It is therefore obvious that selling electricity from solar powerplants in India is not so commercially viable, but selling fresh water generated by solar electricity would be most viable commercial proposition. Also, whatever the growth of terrestrial Solar Power Plants as a result of fall in prices, in the long run the growth may be limited by the following constraints:

Availability of useable and litigation free land space for installation
Inability to meet base load requirements
Inadequate cost effective storage for supply on 24 x 7 basis, and
Environmental impact in the long run due to very large scale installation of PV array structures at the time of decommissioning the plants

Seawater Desalination Using Space Based Solar Power Plants There are several advantages of positioning a solar PV plant in space beyond the increase in its capacity utilization due to absence of the day-night cycle. The solar flux incident on the SPV array (1.42 kW/sq.m.) is higher compared to 1.0 kW/sq.m on earth. This incident solar flux reduces with latitude whereas space solar power intensity is equal at all latitudes. Due to day-night cycle solar water pumps energized by solar energy operate just 5.5 hours in the day whereas they would operate 24 hrs a day when energized from space; solar flux from space is constant throughout day and night, whereas the flux on earth varies from sunrise to sunset; solar energy from terrestrial solar panels is obscured by clouds and rains, dew and moisture whereas the obscuration when beamed from space is much lower. However the overall efficiency of the space solar power system is lower than that of the terrestrial system These factors are analyzed in Table 2 below

Table 2

Capacity Utilization of Solar Panels on Ground and in Space

Sl.No / Capacity Utilization Factors / Value for Ground Based Solar Arrays / Value for Space Based Solar Arrays / Factor
Ratio
(Space/Ground)
1 / Solar Flux Incident on PV Array / 1.0 kW/m2
(Along the Equator, and Reduces with Latitude) / 1.42 kW/m2
(Unaffected by Latitude) / 1.42
2 / Energy availability (Time) for Conversion to Electric Power / 10 hours / 24 hours / 2.4
3 / Average Solar Power Output from a 1Kwp Plant / Average Solar Power Output (in India)
= 0.55 kW / Average Solar Power Output (at all latitudes)
1.00 kW / 1.82
4 / Obscured partially or fully by Clouds and Rain: Non-Sun Factor / Non-sun factor=0.96
Obscured 15 days in a year / Available 12 months in a year
(Satellite Eclipse of a few hours in a year is ignored) / 1.04
5 / Obscured by dust, dew, moisture / Panels clear 97% pf the time (with proper maintenance) / Panels clear 100% of the time / 1.03
6 / RO Plant Utilization due to Day-Night Cycle / SWRO Pumps operate maximum efficiency 5.5-hours /day / SWRO Pumps operate 24 hours /day / 4.36
7 / SPV Overall System Efficiency / 0.72 / 0.52 / 0.72
Overall Increase in Capacity Utilization Factor
(Space relative to ground base solar arrays) / 1.42 x 2.4 x1.82 x1.04x1.03x4.36x 0.72=20.86
or nearly 20 times greater capacity utilization by use of SSP

Capital Cost Advantage of SSP Over TSP Due to the combined effect of these factors listed in Table 2 above, not withstanding the fact that the overall system chain efficiency of space solar power generation (0.52) is less than terrestrial solar power generation efficiency (0.72), the power Capacity Utilization (CU) of solar energy from panels located in space in space is more than 16 times that on earth, i.e. the Capacity Utilization Factor, termed as CUF,is

CUFssp/tsp = 20

However, the cost of space qualified solar cells, the cost of energy conversion to microwave/laser, power transmission and reception technologies, ground receiving “rectennas” etc and above all the cost of space infrastructure (namely, space transportation and assembly in space) are extremenly high and these are not required for terrestrial solar plants. So long as these added costs remain within the added capacity utilization factor, the cost of energy produced on earth and space would be the same.

The cost of terrestrial solar power (TSP) in India is currently $1.6 million/MW and space solar power (SSP) is completive with TSP even when the capital cost of space solar power is as high as 1.6 x 20 = $ 40 million/MW. Only if SSP capital cost exceeds about $40 million/MW does SSP becomes less competitive than TSP for seawater desalination using solar energy.

This is a huge capital cost advantage and will enable economic space transportation even with the high cost of access to space entailed using currentl;y available multistage, vertically stacked, expendable space rocket launchers of the world. Hence space solar systems programmes can be started up right away without waiting for advanced reusable spacetransportation emerging in the next 2-3 decades

So long as capital cost of Space Based SPV system doesnot exceed about $40 million, the cost of fresh water output based in space solar power would always be less than the cost of fresh water produced by a terrestrial solar plant. This 20-fold capital cost advantage however is greatly reduced by the RO Plant Utilization Factor Ratio (indicated in sl. No 6 of Table 2) when space solar power is used entirely for production and sale of electric power

Terrestrial solar power stations are, however, better suited for small-scale decentralized applications whereas space-based solar power stations more suited for large scale base-load applications. As TSP efficiencies increase and as costs decrease, the SSP will find it increasingly difficult to compete with SSP even for large scale applications unless there are niche applications where TSP becomes non- competitive with SSP. Seawater desalination with SSP happens to be one among these competing niche applications for SSP. The SSP and SWRO plants can be built in smaller modules. Thereafter capacity build-up to larger scales can be easily continued to satisfy specific demand when commercial viability is fully established

A recentInternational Academy of Astronautics (IAA) Report[2] suggests a SSP Pilot Plant at more than 10 MW as a first system development venture. However, since solar electric power converted by SWRO into fresh water greatly amplifies the economic value added to electricity sales alone, a more conservative target of 5 MW may be selected as a first-ever techno-commercial pilot plant project.

A small 5MW SSP positioned in geocentric equatorial orbit, when integrated with a 20 MLD (million litres per day) SWRO plant on earth may also find immediate applicationfor consumers along India’s 7000 km long coastline; inremote island territories. In an emergency the 5 MW solar power satellite can be used as Emergency Cooling Water Power back-up safety system for coastal nuclear power plants. The potential total revenue from a 20 MLD SWRO plant has been estimated to be about $ 7061 million (about $7.0 billion) over a SSP-SWRO system lifetime of 20 years. The total expenditure on the proposed STS-SSP-SWRO infrastructure build-up project is thus to be seen and evaluated within this total revenue inflow from the SSP-SWRO pilot plant programme and the surplus available may indeed support R&D into advanced reusable space transportation systems

India is currently facing serious water as well as energy crises. A small scale Proof-of-Concept Technology Demonstrator for Terrestrial Solar Power based SPV-SWRO system (500 litres per day fresh water at 500 ppm output from seawater at 35,000 ppm) has been set up and had gatheredresearch data for nearly 6 years at the Indian Institute of Technology, Madras. The experience gained on thisRig was to be used to design and build a 5000 litre/dayexperimental research facility at the Institute, andthereafter an industrial scale pilot plant.

This paper therefore suggests it is timely and essential that a comprehensive international techno-economic feasibility study be carried out and a small scale technology demonstrator be built for seawater desalination using Space Solar Power

References

[1]M Kumaravel M, R Gopalaswami, K Sulochana and G Saravanan G “Solar Photo Voltaic Powered Seawater Desalination Plants and their Techno-Economics.” Page 1406 , Table 4 , Row 2 ,Scenario B, Proc. of International Conference ISES Solar World Congress (SWC) 2007Beijing, China, Sep, 2007, Vol III, p 1402-1408

[2] John C Mankins, Editor, “IAA Study of Space Solar Power” First International assessment of Space Solar Power, Opportunities, Issues and Potential Pathways Forward, International Academy of Astronautics (IAA) , August 2011