Article by Hans Schnitzer, published in Scheer, H. / Ghandi, M. / Aitken, D. / Hamakawa, Y. / Palz, W. (Ed.): The Yearbook of Renewable Energies 1995/96: Solar Energy – What are its Driving Forces ?, London, 1995

The concepts and results presented here are the outcome of research work and studies carried out within recent years at the Institute of Chemical Engineering at the Graz University of Technology, Austria, in cooperation with the research company STENUM, also in Graz. The various tasks have been undertaken either on behalf of authorities and companies or as independent research.

Since its inception in the year 1990, the concept of an energy System for industrialized countries based exclusively on renewable forms of energy, turned from an exotic, utopian vision into a real and seriously discussed postulate and alternative.

The following article tries to link together the reasons for such a Solution, the question of technological possibilities, the question of costs and some of the necessary actions. Detailed calculations are not included and can be taken from other publications from the Institute.

A Solar-Based Energy Supply System: Why and How?

'Sustainable development' is the one of the keywords to help us overcome the economic and ecological crises of the end of the 20th Century. We are looking for: 'development that meets the needs of the present without compromising the ability of future generations to meet their own needs'.1

What is it that could compromise future generations' ability to meet their own needs? There are three relevant focus points:

- a scarcity of cheap, easily accessible resources (water, energy, metals, etc.): resultant disputes and even wars over them
- a destroyed environment: a great effort to preserve and / or reconstruct the clean environment
- the problem of finding enough Jobs and income for a growing number of people.

A sustainable economic System will be oriented towards these three scarcities - nature, resources, employment - and build up structures to avoid them in time. 'In time' means now; today the right direc-tion must be chosen so that a catastrophe can be avoided.

There is no doubt that the industrialized countries have to start with a modification of their economic System. The necessary knowledge can be found here, as can economic power and freedom of choice, without the Situation being driven by a starving population. Here, in the industrialized countries, a minority of the world's population is needlessly consuming the majority of the world's natural re­sources, causing the major emission and waste Prob­lems. Here there is the chance to set examples to the developing countries in the East and South, even if we have to accept that each society has to learn by its own mistakes.

The earth is - seen from a thermodynamic point of view - a closed system. Like in the steam cycle of a power plant or in the refrigerant cycle in an air conditioner, energy flowing to and from the System drives mass within it. While huge amounts of energy are insolated from the sun to the earth and are radiated off again into space, the mass exchange is negligible. Energy is coming from the sun - a hot radiator of about 6000K - in a highly valuable form and is transformed by a number of processes into a low-value form - 'waste'" heat of about 290K (17°C). This helps us to keep the entropy balance even; all the entropy produced on earth is exported with this low-quality energy. The surface temperature of the earth is regulated by the energy balance and the greenhouse effect of the atmosphere. Through the absorption and reflection abilities of various gases in the atmosphere, the earth's surface is warmer that it would be without.

This heat balance is now seriously influenced by human-caused emissions of large amounts of gases, leading to increased global warming. The main gases responsible for this effect are carbon dioxide (CO,) and methane (CH4). At the moment, it can be accepted as certain that they lead to a change in climate. CO, emissions mainly originate from incineration proc-esses, bringing carbon from fossil stores into the atmosphere.

So the reasons to switch over to a solar based energy System are numerous:

- to avoid the risk of a change in climate caused by greenhouse gas emissions
-  the shortage of non-renewable energy resources
- the basic postulate of sustainable development for closed material cycles.

Besides these preventive arguments, there are short-term incentives to starting the changeover:

- the switch to renewable forms of energy offers a great opportunity for our agriculture and forestry as well as for industry and commerce in the field of new products and new jobs
- ahead lies an opportunity for companies to expand into markets in cases where this unavoidable development starts later.

Availability of Solar Energy

Solar energy is not to replace fossil energy, but to provide the energetic services that are asked for by society. It is therefore not necessary to provide an unchanged amount of energy, but all demands and uses have to be covered.

Table 1. Order of magnitude of yearly global energy fluxes  2

Table 1 gives an insight into the amounts of energy that are in action on the earth during a period of one year (1TW (terawatt) = 10'2W) 2.

It is obvious that the energy received surpasses the demand by a factor of 10,000. At the same time it can be seen that the production of biomass is greater only by a factor of 10.

In Austria - as an example of an industrialized country in a mild continental climate -the average solar radiation is 1050kWh/m2 /yr, corresponding to about 1000 hours of füll sunshine with 1000kW/m. Covering an area of 84,000km2, Austria receives 88.2 million GWh or 317,500PJ (petajoules, 1015) per year. This stands in comparison with a consumption of 864PJ/yr (1992). Table 2 gives some figures rel­evant to the development of a solar-based energy System in Austria.

Table 2. Relevant figures for a solar-based energy System in Austria

It can be seen from Table 2 that the area required for a solar-based energy System can be found easily if an average conversion efficiency of 10% is assumed. Similar calculations can be made for other countries, leading to similar results.

Solar Paths

After having proved that enough solar radiation is available, the question of available technologies must be discussed. In general, each path from solar radia­tion to the energy supply consists of the following three steps:

receiver / conversion transport / storage utiliza­tion / conversion for supply

Figure 1 shows in a schematic way how solar radiation can be converted into energetic Services through a number of different paths. This figure includes the routes to raw materials for chemicals, since crude oil and natural gas have to be replaced here too.

Figure 1. Major paths for the utilization of solar energy

Biomass and hydro power are technologies already widely used today. The utilization of local tempera-ture (dT) and concentration (dc) differences is still at an early stage of development.

The various technologies differ in the following respects:

- specific demand for area
- form of energy delivered (regarding transport and
- storage possibilities)
- demands on the land (ground, fertility)
- need for auxiliary materials and manpower in
- construetion and operation. Table 3 gives an overview of how much energy can be obtained by area (m2) via the various technologies (area Performance of energy carriers). For auxiliary energy use (pumps, transport, saws, etc.) - which have to come from solar sources too - further area is needed. As there are two possible routes - 'technical' (wind, hydro, PV) or 'biomass' power plants - the effective area Performance is lowered. In some cases, the utilization of production 'wastes' such as straw can increase the total Performance in comparison to that obtained regarding one single product.

From this table it is not yet possible to calculate the area needed for a completely solar-based energy system, for various reasons. On the one hand, there are various routes from the radiation to the service required. Thus room heating can be obtained by burning wood, through heat pumps using electricity from hydro power and through direct use and storage of solar energy. On the other hand, the minimum energy demand for the energetic Services is not known in most of the cases. So there is no minimum amount of energy that is needed for heating a house or providing transportation.

The 'optimum' solution will always be determined by economics, and will be different to the one requir-ing the minimum amount of area. In many cases, e.g. room heating, it will not be possible to distinguish between energy conservation and solar energy utili­zation. The third parameter that can be freely chosen in designing a solar-based energy system, is the allocation of resources. How should wood be used: in industrial steam power plants for high-temperature heat and electricity, for district heating with cogeneration or in gasification? The final decision will be a political one.

Table 3. Performance by area for various solar technologies

Some routes for solar energy to the most important energy Services are described next. All the technolo-gies cited are available today or close to realization.

High-Temperature Heat

High-temperature heat is almost exclusively used in industrial applications. Here we have to distinguish between two types:

- heat up to 400°C that can be supplied by steam or direct burning of fuels and

- heat in the temperature ränge above that, which requires very hot flames or electrical arcs.

Most processes in industry do not surpass 200°C. This is the case for all processes in food technology, pulp and paper, organic chemistry, textiles and leather, and many others. Here we find Operations like cook-ing, evaporation, distillation, pasteurization, sterili-zation, drying and cleaning. Process heat can be provided by burning biogenous materials of all kinds as it is done with fossil fuel at present. Incineration of bark, wood chips and biogenous wastes is well known in many applications right now.

Regarding industrial processes, one has always to keep in mind that the present amount of energy needed can be reduced substantially in sustainable development. Some of the Services provided now are not needed within a sustainable industrial develop­ment. It would no longer be necessary to operate refineries and some of the processes in the chemical industry. Processes based on biotechnology demand lower energy inputs in general, or at least they operate at much lower temperatures than traditional petro-chemical ones so that collectors can be used.

On the other hand, the utilization of natural re-sources might lead to greater complexity in prepara-tion and Separation. It is not yet possible to estimate the final amount of energy that would be needed in industry.

Processes with heat above 300°C are: metallurgical ones in general, glass manufacture or cement kilns. These processes are usually fired by coke or gas, and by electricity in a rising number of applica­tions. Coke is required for technological reasons in some cases and not as an energy carrier alone. (This case does not need consideration here. Firstly, it is of course possible to use charcoal again, as has been done for centuries in history. Secondly, the amount of coal needed for these processes is so low and specific that there is really no serious danger of hurting the principles of sustainability.) All processes heated by the direct use of electricity demand a supply system, which generates electricity through sustainable alter­natives. This will be discussed later.

Low-Temperature Heat

Low-temperature heat (room heating, tap water, etc.) represents the greatest amount of energy needed in
most of the industrialized countries. About one-third of the total energy demand is located here. The routes to this service are manifold and can use a number of possibilities.

It is obvious that solar radiation can be trapped by acti ve and passive devices like Windows, solar collec­tors and transparent insulation materials. It may be difficult to distinguish between energy conservation and solar technology in many cases, especially when we think about architecture. The optimized design of buildings is always a combination of energy conser­vation and passive utilization of solar heat.

Many building designs demonstrate that the heat­ing load can be up to 100% covered by solar collectors and storage devices. These 'fully solar-powered houses' will not, however, be the most important Option in the near future. A partially solar-covered system seems to be much more appropriate at the moment. So is it possible to reduce the energy demand of a well-insulated single-family house by about 50-80% through a combination of a collector system of about 20m² and a hot water storage system of about 10m³.

Low-temperature heat can also be provided by burning biomass. This will be wood in general, but also bark or straw in district heating Systems. Burning biomass can be combined with cogeneration Systems, which produces electricity at the same time. In the pulp industry circulation bed vessels in the pulp industry are a commonly used technique for cogeneration. Newer technologies use hydrothermal pretreatment, enzymatic hydrolysis or gasification Systems for wood, bark or any other liquid or solid biomass.4 Gasifiers can be used for düng (biogas) and activated Sludge from wastewater plants too.

Finally, low-temperature heat can also be supplied through electricity. This alternative is not shown in Figure 1, since nowadays electricity is too valuable for this application. If there is a surplus of electricity some day through the intensive use of cogeneration and photovoltaics, this path will have to be reconsid-ered. Of course, no direct heating should be done, but more efficient technologies have to be used. Here the heat pump system offers great potential, as soon as the other problems with this technology - mainly the ozone layer problem caused by most of the refriger-ants - are solved.

Transportation and Mobility

Mobility for materials and persons is an essential aspect of our present society. This is not the place to discuss the true need for these Services in their present order of magnitude but to look at the problem from the demand side, several issues have to be taken into consideration:

Individual Mobility of People

Several types of mobility needs force people to move between locations:

Between home, workplace and Shopping area. The design of cities, the location of Shopping centres and the trend towards living in the countryside have forced people to spend hours every day in cars. Public transport is not succeeding at present in taking over the major part of this task.

The answer to this problem in a sustainable society is to avoid this need. There are several possible alternatives.

First of all, workplaces, residential areas and Shop­ping centres should again be located more closely together. This would demand smaller units in each of these three fields: smaller shops, smaller companies and distributed residential areas.

Home offices - people who work in their own homes and communicate via electronic devices - will increase in number and importance. In 1994 there were more than 8 million telecommuters in the USA.5. They spend three days a week working at home and two days at the Company. Laws like the Clean Air Act, demanding a reduction of traffic in cities, accelerate such developments. It can be expected that 40% of all US Citizens will be telecommuters in the year 2000. Desk sharing and car sharing could be economic advantages for companies and employees. Telecommunication can also replace some mobility needs through Tele-Shopping. Ordering goods via home Computers and TV sets is already being developed and tested already now. Although some people will not find this a good alternative to going Shopping, many things in everyday life could be settled in this way. This is certainly true for banking and financial Services, for example.

Between cities for business reasons. Again telecommunication can be an important aid to avoiding this requirement. Video-conferences, Computer networks and worldwide phone calls from mobile phones will reduce some of the need for business journeys. Journeys within a few hundred kilometres should use rail transport on rails, powered by electricity, rather than private cars or planes using petrol (regarding electricity, see below).

Long-distance international travel for business reasons. Here the same applies as above, except in most cases for the possibility of using railways. A need for air travel remains, which could be covered by planes powered by hydrogen (in the very long term).

Need for Transportation of Goods

There are several reasons for the strong growth in transportation in recent years. One of them is that many companies have decreased the ränge of their functions. Many operations, e.g. painting, cleaning, production of small parts, are transferred to other specialized firms. This makes economic and some-times also ecological sense, since those companies should be at a higher level of technology and environ­mental protection in general. But the need for trans­portation detracts in part from the emissions saved. Changed costs for transportation energy and for man-power in a sustainable society - which will certainly occur - will lead to a number of modifications.

- Changing production patterns by switching over to smaller units and a more holistic approach to production offers the possibility of reducing the demand for transportation.

- For transportation in urban areas electric-driven vehicles are already on the market. Traffic restric-tions for combustion engine driven cars areas might accelerate this approach. Of course, elec­tricity has to be provided according to the princi-ples of sustainability.

- Today, transportation over long distances is most effective and economic on the road by trucks or by air transport Systems. However, rail Systems and boats offer a much better Solution in terms of environmental protection and should help to avoid the need for liquid fuels.

Stationary Power Demand

Many customers need stationary power. We find possible applications everywhere, from households (mixers, air conditioners, refrigerators, cookers, etc.) to industrial ones like conveyor belts and all kinds of machines and drives.

Electricity will remain the main power supply in a sustainable society. Alternatives like combustion engines with gaseous or liquid fuels from biomass should be possible, but are too expensive and cause local emissions.

There are several routes to generating electricity from solar energy.

- The best known alternative at present is hydro power. Sunshine, causing evaporation of water, wind and rain, has been used in the form of hydro power for centuries. There are, though, two limi-tations to the extended use of this technology: the limited availability of rivers and the impact of (large) hydro power plants on local ecosystems and biddiversity. Not enough is known about possible changes in the climate caused by dams, but projects in Egypt and the former Soviet Union clearly demonstrate that this has to be investigated.

- Burning solar-originated biomass in power plants is a second route to electricity. Several units of this type are in Operation in industry and in some district heating Systems. Cogeneration with steam turbines will be used in these cases, but also gasification and Stirling engines.

- Photovoltaics will become cheaper in the near future and offer a further possibility of generating electricity.

- There could be new technologies on the market in future like thermoelectric and chemoelectric systems.

Light and Information

Light and Information (TV, computers, telecommunications, etc.) are important services to society, but do not account for the use of very much energy, only a few percent. For these applications, there are prac-tically no alternatives to electricity.

Supply of Total Energy Demand through Solar Technologies

It is not a new idea to discuss the possibilities and Problems of covering the total energy demand of an industrialized country by solar energy.6 As discussed above, there are a number of alternatives for provid-ing the Services needed. Table 4 shows the land required in Austria, assuming different technology mixes. It is not assumed that savings reduce the need for energy; the conservation potential should be used to stop the growth in energy consumption. No change in the mobility sector is assumed either (same amount of private and public transportation). For today's use of biomass and for hydro power no area is calculated. A small further amount of biomass can be used without agricultural effort by using waste materials (+ 130PJ/yr). It is assumed that seasonal storage Systems for low-temperature heat are available (they are accounted for through efficiency factors).

Table 4. Area required for a total solar energy supply in Austria

1 Energy supply with solar cells, excluding utilization of hydro power and biomass 2,200

2 Energy supply with solar cells, including utilization of hydro power and biomass as used today 1,520

3 Energy supply through solar cells; low-temperature heat with thermal collectors; hydro power and biomass as used today 1,290

4 Energy supply through solar cells; low-temperature heat with collectors; hydro power +10% and biomass + 130PJ/yr 925

5 Energy supply based on H2 with solar cells and electrolysis; low-temperature heat with collectors; hydro power and biomass as today 2,420

6 Energy supply based on H2 with solar cells and improved electrolysis; low temp. heat with collectors; hydro power and biomass as today 2,050

7 Energy supplied only through biomass (forests), but including hydro power as today 92,500

8 Energy supplied only by biomass using new agricultural techniques and including hydro power as today 51,000

9 Additional available area, calculated from a questionnaire among Austrian energy experts 6,488

Altogether, the assumptions are rather conven-tional and all of the technologies are available or will be developed in the near future. Better efficiencies will reduce the size of land required: in particular, a rise in the annual efficiency of solar cells from 7% to 8% will reduce the need for land by 14%.

An energy System based completely on biomass requires an area that cannot be provided in industrial­ized countries, due to the low efficiency factor of the conversion of solar energy via biomass to electricity or heat (0.2-1% only). Even planting fast-growing plants that might produce ten times more biomass per m2 and year would not really solve that problem. After all, sustainable agriculture must be guaranteed. Biomass cultures require high-quality soils, where food could be grown too. Only very few plants that show high yields are native in Central Europe and can be grown without fertilizers. Hemp might be an example, but is forbidden in most of the countries

Economic Aspects

Energy is extremely cheap at the moment. While in 1950 it was possible to buy 2.4 litres of gasoline with the wage from one hour's work, nowadays 15 litres can be bought. Only for very few companies is energy an expensive factor.

If one carries out a total cost analysis of conven-tional and regenerable energies over the füll lifetime of an installation, including all additional charges, it turns out that even today many alternatives are cheaper or at least not more expensive. In carrying out a comparison, investment costs (money/kW, money/ kWh/yr) of all installations, including storage facili-ties and Operation costs (money/kWh), are to be compared. Most solar technologies have high invest­ment costs and rather small operating costs. So they might not be economic in the short term, but deliver cheaper energy over a whole lifetime.

Figure 2 shows those technologies that already display lower energy costs, as soon as a total cost analysis is performed. The comparison is based on the true costs of new installations for average applica­tions. The basis for the comparison is the technology most commonly used at present (oil, coal, gas, etc.) The lifetimes for the installations differ (15 years for machinery, 30 years for buildings, etc.).

One can see from this figure that a number of Steps in the direction of sustainability through the use of renewable energies could be made now. Others - e.g. photovoltaics - still need further development before becoming advantageous.

Need for Research and Development

In principle, the technologies are available to provide all the energy needed in an industrialized country through solar energy. In reality, the great majority of our technical demand is covered through fossil forms of energy such as gas, oil and coal.

Figure 2. Comparison of total costs and operating costs for economic alternatives to conventional energy systems

Some industrialized countries in Europe, such as Austria, Norway and Switzerland, cover a remarkable fraction of their electricity demand with hydro power, the most commonly used solar technology.

To increase the use of renewable energies further, certain measures need to be taken:

•        more information

•        technical development

•        changed economic environment

Regarding the technological development there are three options:

•        improving the efficiencies of existing technologies (e.g. photovoltaics)

•        improving the economic Performance of existing technologies

•        research in and development of new technologies.

The main obstacle to the extensive utilization of solar technologies at present is the low economic Perform­ance. It is therefore the most important task for researchers and industry to improve the energy pro-duction per unit of money. This is much more impor­tant than improving the energetic efficiency (per unit of area).

Much potential seems to be involved in the devel­opment of better Systems, rather than components. The Integration of photovoltaics into the electricity grid and into architecture 8 in order to avoid construc-tion costs, might support development more than increased efficiency of the photovoltaic cells.

Policy Challenges

The next Steps towards sustainable development re-quire very clear decisions from policy makers. The detailed analysis of the possibilities for a fully solar-powered energy System for industrialized countries

showed that this is feasible. We have at present the technologies to provide such a System. We also know that the costs of such an alternative can be easily covered.

Politics is very reluctant to decide on such a course. Of course, there are uncertainties left but, on the other hand, the problems that will occur if we continue the way we are going now are obvious too.

To take the next Steps towards a sustainable energy System, the following actions have to be taken:

- Change building Standards in the direction of better- insulated houses (minimum energy houses) in order to reduce the energy demand in the most important sector.

- Change the tax System to make fossil fuel more expensive (CO, tax).

- Make solar collectors obligatory for water heating in summer to avoid the Operation of small boilers at this time of the year.

- Support cogeneration plants in industry and dis-trict heating - especially if based on biomass or other renewables - to replace fossil-fired power plants without cogeneration.

- Support research and development in regenerable forms of energy and in energy conservation ac-cording to their possible contribution to sustain­able development.

- Support research in life-cycle analyses regarding solar technologies to ensure the quality of the development.

Although it is not absolutely clear what sustainability will look like regarding energy, many Steps in the right direction can be done now. It is absolutely certain that overall reduction of the amount of energy consumed is essential. More than this, all forms of solar energy - collectors, photovoltaics, hydro power, biomass, wind, etc. - have to be utilized in a network.


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3. Schauer K, Schnitzer H. (1994). Solartechnologien in Österreich zur Minimierung der C02 Emissionen. Bundesministerium für Umwelt, Jugend und Familie, Vienna.

4. Federal Ministry of Science and Research (1994). Energy from Biomass - R&D in Austria. Vienna.

5. Lotter W. (1994). Einsame Masse, profil 25, 17, 44.

6. Nakicenovic N, Messner S (1982). Solar Energy Futures in a Western European Context. Working Paper. IIASA, Laxenburg, Austria.

7. Herer J. (1993). Hanf. Zweitausendeins. Frankfurt.

8. Humm O.Toggweiler P (1993). Photovoltaics in Architecture. Birkhäuser, Basle.