Earth’s Future

Electric car with solar and wind energy may changethe environment and economy: A tool for utilizingthe renewable energy resourceQuanhua Liu

ESSIC, University of Maryland, College Park, Maryland, USA

Abstract Energy and environmental issues are among the most important problems of public con-cern. Wind and solar energy may be one of the alternative solutions to overcome energy shortage andto reduce greenhouse gaseous emission. Using electric cars in cities can significantly improve the airquality there. Through our analyses and modeling on the basis of the National Centers for EnvironmentPrediction data we confirm that the amount of usable solar and wind energy far exceeds the world’s totalenergy demand, considering the feasibility of the technology being used. Storing the surplus solar andwind energy and then releasing this surplus on demand is an important approach to maintaining uninter-rupted solar- and wind-generated electricity. This approach requires us to be aware of the available solarand wind energy in advance in order to manage their storage. Solar and wind energy depends on weatherconditions and we know weather forecasting. This implies that solar and wind energy is predictable. Inthis article, we demonstrate how solar and wind energy can be forecasted. We provide a web tool that canbe used by all to arrive at solar and wind energy amount at any location in the world. The tool is availableat www.renewableenergyst.org. The website also provides additional information on renewable energy,which is useful to a wide range of audiences, including students, educators, and the general public.

1. Introduction

Total carbon dioxide (CO2) emission from fossil fuel burning reached 28 billion metric tons in 2005 andthe emission has resulted in the increase of CO2 concentration in the atmosphere [International EnergyAgency, 2008]. CO2 is called a greenhouse gas because it traps long-wave radiation escaping to space andmay be responsible for the increasing global mean surface temperature [Cotton and Pielke, 1995; Kiehl andTrenberth, 1997]. Recent decrease of snow and ice quantities over the Arctic [Oelke et al., 2004] and thecollapse of a huge ice chunk in western Antarctica in March 2008 have brought more public attention toclimate change [Houghton, 2001]. Fossil fuel burning has also led to acid rain and atmospheric pollution.Air quality of cities is mainly affected by vehicle emissions. When Beijing restricted the use of personalvehicles (a car with an even-numbered tag may be used on even dates) during the 2008 Olympics, airquality was significantly improved [Wang et al., 2010]. We can imagine how good air could be if everyoneused electric vehicles. A shift from gasoline to electric cars would greatly boost the economy if we speedup the switchover to electric cars. Certainly, more electric cars would mean fewer gasoline cars needed.In addition, a faster shift to electric cars will need more new cars than before, so more new jobs will beadded. This article discussed a future perspective of zero-emission electric vehicles derived by solar andwind energy. Solar and wind energy resources can meet the world’s demand. The total annual downwardsolar energy at the surface is 6800 times more than the world annual energy consumption. The globalwind within 200 m of the surface provides much more power than current global power consumption.Accurate estimation of global wind power is complex because the capture of wind power in one placemay affect wind power in another place.

Solar and wind energy has been used for a long time. Solar heat has been used to dry foods, cook food,and heat water. Solar electricity has also become a unique energy resource on spacecrafts. Photovoltaic(PV) cells can convert sunlight into electricity. Solar thermal technologies use concentrator systems toheat a working gas or fluid for running a conventional power plant [Forsberg et al., 2007]. Windmills havebeen used to convert wind energy into mechanical energy for more than 1000 years. However, renewableenergies are still highly underutilized. A paper [Turner, 1999] in Science Magazine outlines the future of

RESEARCH ARTICLE10.1002/2013EF000206

Key Points:• Solar and wind energy are clean and

renewable• The renewable energy can be

converted into electricity• Electric cars powered by the

electricity will improve air quality

Corresponding author:Q. Liu (qliu123@umd.edu)

Citation:Liu, Q. (2013), Electric car with solar andwind energy may change theenvironment and economy: A tool forutilizing the renewable energyresource, Earth’s Future, 2, 7– 13,doi:10.1002/2013EF000206.

Received 30 SEP 2013Accepted 19 NOV 2013Accepted article online 9 DEC 2013Published online 16 JAN 2014

This is an open access article underthe terms of the Creative CommonsAttribution-NonCommercial-NoDerivsLicense, which permits use and distri-bution in any medium, provided theoriginal work is properly cited, the useis non-commercial and no modifica-tions or adaptations are made.

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renewable energy. The technology for solar and wind energy is now ready for applications [Grimes et al.,2007], but the cost is very challenging if environmental issues caused by fossil fuel burning are ignoredand the Earth’s treasury is never paid. In addition, to provide uninterrupted cost-effective energy supply,the storage of solar and wind energy is necessary.

In this study, we analyzed 65 years (1948–2012) of reanalysis data from the National Centers for Environ-ment Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) [Kistler et al., 2001]. It isa 2.5∘ × 2.5∘ in latitude and longitude gridded data set. The data set is used for studying the global trend.The NCEP T384 (∼50 km resolution) analysis data set that is of a higher spatial resolution and used forimaging is used in our web tool. The downward solar radiation flux is obtained from the data sets.

Air temperature, surface pressure, and wind speed are used to calculate the surface wind power.

2. Solar Energy

The whole Earth, which has a cross section of 127,400,000 km2, approximately receives solar power of1.740× 1017 W and reflects about 30% of the power back to space. The annual incoming solar radiationat the top of the atmosphere is about 5.5× 1024 J, and 60% of the energy reaches the surface. The totalannual downward solar energy at the surface is about 3.3× 1024 J, which is 6800 times more than theworld’s annual energy consumption. Even by excluding the water surface (70%) and assuming a solarenergy conversion efficiency of 10% [Rühle et al., 2009], the usage of solar energy over 0.5% of the landsurface can meet the current global energy demand. The downward solar radiation at the surface dependson time, latitude, atmosphere, aerosols/clouds, and surface conditions.

Solar energy can be converted directly or indirectly into other forms of energy, such as heat and electric-ity. Through vegetation photosynthesis, solar radiation can transfer CO2 in the atmosphere into biodiesel.PV energy results from the conversion of sunlight into electricity through a PV cell, commonly called asolar cell. The PV cell uses a part of solar radiation for generating electricity. Only a photon having enoughenergy (its wavelength< 1120 nm) can free an electron from the silicon, generating current for electric-ity. Solar thermal technologies use concentrator systems because of the high temperatures needed forthe working gas or fluid. A parabolic trough and solar dish are two examples of efficient concentratorsystems [Duffie and Beckman, 2006]. The world’s largest parabolic trough facilities, located in the MojaveDesert near Barstow, California, produce 354 MW of power at peak output. In the solar energy plant, oilis used as the carrying medium to absorb solar thermal energy and goes through a heat exchanger togenerate steam, which is in turn used to run a conventional power plant. We may consider an alternativemodel such as a combined natural gas/steam power plant as a solar energy plant. The combined naturalgas/steam plant achieves a conversion (from thermal to electric energy) efficiency between 50% and 60%[Bachmann et al., 1999], which is higher than that of a single steam plant, which is between 30% and 40%[Duffie and Beckman, 2006]. The solar beam/steam plant would be the same as the conventional com-bined natural gas/steam plant except that the fuel gas for firing the combustion chamber will be replacedwith solar heat.

Solar energy as a resource at high latitudes is poor. It is also low over the Amazon rainforest owing to thelarge absorption and scattering of clouds there. However, solar radiation over deserts and bare soils at lowlatitudes is rich. For example, the Qinghai-Tibet plateau may serve as a vast solar energy base for China.A number of locations in deserts and other nonvegetated land regions have been proposed for poten-tial solar energy applications [Service, 2005; Liu et al., 2009]. Additional factors, such as population, localenergy demands, and the cost of energy transportation, need to be considered when selecting a loca-tion for a solar power plant. It needs to be pointed out that only a very small area of each proposed site isneeded for the actual plant.

3. Wind Energy

Wind energy is the atmospheric kinetic energy determined by the mass and motion speed of air. The spa-tial gradients of the absorbed solar radiation by the Earth-atmospheric system are the primary source fordriving atmospheric and oceanic flows and transferring heat from one part of the globe to another. Thisatmospheric motion contains huge kinetic energy that can be used to benefit mankind. The kinetic energy

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K for an air mass m with a wind speed V can be expressed as:

K = 12

mV2 (1)

The air mass can be derived from the product of its density and volume. For a constant wind speed of Vand normal section area of A during a given period of time t, the air mass can be expressed as:

m = 𝜌AV t, (2)

and the kinetic energy of the air mass is:

K = 12𝜌AtV3

. (3)

The wind power density (power per area, W/m2) [Gipe, 2004] is:

Power density = 12𝜌V3

. (4)

The atmosphere approximately observes an ideal gas equation in which air density at the standard tem-perature (T 0) of 288.15 K and at a sea-level pressure (P0) of 101,325 Pa is 1.225 kg/m3 [Gipe, 2004]. There-fore, the air density (kg/m3) for a temperature T and pressure P can be written as:

𝜌 = 𝜌0P

P0

T0

T= 1.225 × P

101, 325288.15

T. (5)

Using equations 4 and 5 we can derive the wind power density at a temperature T (in Kelvin) and at pres-sure P as:

Power density = 0.61125 × P101, 325

288.15T

V3. (6)

Therefore, the wind power density is proportional to a cubic law of the wind speed. Because of the nonlin-earity and the distribution function (e.g., Rayleigh distribution) describing the frequency of occurrence forthe wind speed, an energy pattern factor (Epf ) [Gipe, 2004] is applied when an averaged wind speed Va isused to compute the wind power density. That is

Power density = Epf × 0.61125 × P101, 325

288.15T

V3a . (7)

In this study, an energy pattern factor, Epf= 1.91 for a Rayleigh distribution [Gipe, 2004], is applied whenan averaged wind speed is used to compute the wind power density.

4. Solar and Wind Energy Forecasting

One of the most challenging issues is how to provide constant year-round solar and wind energy dayand night. In addition to location, solar and wind energy is weather and time dependent. No or littlesolar radiation can be used during night time and overcast/rainy day and wind turbines cannot operatewhen there is no wind or when wind speed is too high (such as hurricanes). Therefore, it is necessary tostore surplus power on good operational hours/days and release it at other times, thus making the poweravailable on demand. This strategy has already been applied through the storage of potential energyby pumping water uphill to a reservoir using wind power. A recently published New York Times article[Matthew, 2008] described an idea for storing the sun’s heat using cost-effective storage. With a well-insulated reservoir, heat can be stored for hours or even days. Molten salt may be an effective low-costsolar heat battery (http://www.treehugger.com/files/2008/01/molten_salt_as.php). Methods of efficientstorage by themselves are, however, not sufficient. It is equally important to estimate, plan, and managethe energy need. The current data available from our website provide long-term and general distributionof solar/wind energy and should be enough for strategic storage planning. But short-term forecasting ofthe actual energy availability is critical to the scheduling of electricity production and storage.

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Figure 1. Errors in solar power forecasts as a function of forecast length. The black and dashed lines represent 3 h and daily means,respectively.

We can predict, to a certain degree, solar and wind energy indirectly based on weather forecasting. Inthis study, we use the NCEP global forecasting data to estimate the accuracy of predicting solar and windpower. The bias and root mean square (RMS) error are determined by comparing the calculated solar andwind power using the forecast fields and using analysis fields at the forecast time. The analysis fields areproduced by assimilating observations including satellite radiance measurements and a priori or fore-cast information under dynamic constraints following a set of physical laws. The analysis fields are usuallytreated as truth to analyze forecasting errors. We estimate the forecasting error for solar and wind energyby using 5-day forecasting data in January 2008. The bias is very small at the global level, which is notsurprising because weekly variation of the globally averaged solar and wind power is small. On the otherhand, local variation of solar and wind power can be quite large owing to weather changes. Figure 1 showsthe global mean of the relative error (RMS error/mean value) of solar power forecasts as a function of fore-cast length. For a 24-h forecast of a 3-h mean, the relative error is about 30%, and for a prediction of thedaily mean, about 15%. These relatively large errors are mainly due to inability of current models for pre-dicting clouds accurately. For a 24-h forecast of a 3-h mean, the relative error of wind power forecasts isabout 70%, and for a prediction of the daily mean, about 35% [Liu et al., 2009]. Wind prediction errorsare due to the extreme temporal variability of wind speed and the inability of models to capture suchfine-scale structure. The forecasting accuracy for solar and wind energy will be improved as weather fore-casting is improved. Over the last three decades, the accuracy of NWP forecasts has improved at a rateof about 1 day/decade, i.e., today’s 5-day forecasts are as reliable as 2-day forecasts were 30 years ago[Richardson, 2000]. Further gains are anticipated with better physical models, more powerful computers,and better observations, especially from space.

5. Electric Car

The electric car was a popular car in the early twentieth century and declined in the market after WorldWar I because of massive discoveries of crude oil and cheap gasoline. After 100 years, the electric car iscoming back into the market. Today we are facing depletion of crude oil supplies and increasing CO2

in the atmosphere by fossil fuel burning. Total CO2 emission from fossil fuel burning reached 28 billionmetric tons in 2005, and the emission has resulted in the increase of CO2 concentration from 280 to 396ppmv in the atmosphere. As a result of rising prices of gasoline and concern about climate change allover the world, more and more people are starting to look for alternatives, such as the electric hybrid carand pure electric car. A 100% Electric Zero Emissions Vehicle (EZEV) was exhibited for sale at Baltimoreharbor, Maryland. Many people stopped and were attracted by the advertisement on its windows: a 100mile (160 km; 1 mile ≈ 1.6 km) range at a speed up to 70 mile per hour and a cost of 70 cents for driving

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Figure 2. Decadal trends in surface wind power (black line) and surfacetemperature (dashed red line).

100 miles in comparison to $14 (4 gallonsof gasoline to drive the same 100 miles)for a car using gasoline. The technologyfor the electric car is feasible and accept-able: 8 years or 100,000 mile warranty and$15,000 for a brand new electric car afterfederal tax credit of up to $7,500.

Although the electric car is promising,its benefit may be exaggerated. Sev-enty cents for driving 100 miles may bean extremely cheap rate, but it cannotbe sustainable when more electric carsare charged at night time. It is better tosay how much electricity is needed for

driving 100 miles. According to the features and specification of the manufacturer, 24 kWh of electricity isneeded for driving 100 miles for a THINK City or a Nissan LEAF. According to the utility bill for residents inMaryland, 24 kWh of electricity costs about $3 instead of 70 cents in the advertisement mentioned above,thereby taking into account the cost for electricity generation and grid transport. Thus, zero emission isnot true right now. Producing 24 kWh of electricity will consume about one and a quarter gallons of gaso-line. Therefore, the equivalent efficiency for the pure electric car is about 90 miles per gallon, which ishigher than any other car including hybrid cars.

6. Acquiring Solar and Wind Resource

With advanced Google map technology, one can easily acquire from http://www.renewableenergyst.orgthe amount of solar and wind energy available at any location or area in the world. On the solar energypage, the map in the upper-left corner provides a simple interface. One can either roam to a location,type in a location name, or type in an address and then use the zooming and drawing buttons to drawan area on the map. The total solar radiation available for power generation in that area will be shown. Bymultiplying a conversion factor, typically between 0.1 and 0.2, one can figure out how much solar powerone may get using a solar power device. If a user types in his/her own home address, zooms in to his/herbackyard, draws an area in the backyard, and provides a conversion factor value (e.g., 0.15), the user willfind out how many kilowatt hours of electricity can be generated from a region in the backyard.

Similarly, one can also find from the wind power page the amount of wind energy for any location in theworld. Total available wind energy also depends on several additional parameters, including the rotor’sdiameter, the number of rotors to install, and the height of each rotor. The number of rotors is estimatedfrom the size of the area and each rotor’s diameter. By multiplying an efficiency factor, typically between0.15 and 0.40 (default value 0.2), a user can figure out how much wind power can be obtained from windturbines. Wind energy increases rapidly with height for a constant flux layer. We use a power law to calcu-late wind speed at a given height from an available wind speed beneath the height. The power law maybe applicable below 100 m, but a different parameter value in the power law may be needed when theheight is above 100 m. Thus, it is possible to get much more energy by setting a wind turbine at a higheraltitude. For instance, wind energy density is doubled when a wind turbine is moved from 10 to 50 mabove the surface.

7. Discussion

Renewable, sustainable, and clean solar and wind energy resources are much larger than the world’senergy demand. Solar energy from just 1% of the land area in the United States can meet the wholenation’s energy needs. Large wind energy resources are located over the oceans and near coasts. Windpower plants can be constructed along coastal areas where energy demands are high. In addition, thereare many inland sites with great potential for wind energy applications. Air pollution is a big issue in thecities for developing countries. City pollution is mainly caused by vehicle emissions. We can have zeroemissions if we use electric vehicles derived by solar and wind energy. Using electric cars should not hurt

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Figure 3. A geographic distribution of decadal surface downward solar radiation change in the past 50 years. The value is thedecadal mean over a period from 1999 to 2008 minus the decadal mean over a period from 1949 to 1958.

economy. Oppositely, the business of electric cars can create a lot of jobs in the areas of new electricitycharge station construction and operation, battery production, renewable energy, new car design, andproduction. Electric cars and vehicles have low emissions (zero emission by using renewable energy)and can become integral parts of a smart grid, where they do not just consume power but also providemobile storage of energy. Electromobility’s greatest potential for climate protection is the interaction ofrenewable energies and with sustainable mobility. Many countries are supporting and working on elec-tromobility. A million electric vehicles are expected to be on the road in the United States by 2015. Withgovernment investment exceeding $140 billion, the United States is hoping that this business can bringmillions of jobs and also dramatically reduce CO2 emission.

Environment, climate, and electronic vehicle powered by solar and wind energy could influence eachother. Using the renewable energy can ease pollution and reduce CO2 emission. On the other hand,climate change will affect solar and wind energy distribution in the future because solar and wind energydepends on weather conditions. To obtain an estimate of the magnitude of these effects, we analyzed65 years (1948–2012) of the NCEP/NCAR reanalysis data [Kistler et al., 2001]. Surface wind power exhibitsa significant positive trend with global warming as shown in Figure 2. The correlation coefficient betweenthe surface wind power and temperature is 0.82. We computed a decadal mean global map of windenergy for the period 1949–1958 and for the period 1999–2008, and then determined the differencebetween the two. It was found, for example, that most of Asia had experienced a decrease in surface windenergy although wind energy over oceans increases significantly [Weng et al., 2012]. We found no signifi-cant trend in downward solar radiation at the surface in the data set. However, we do see a distinguishedpattern change in solar radiation. Figure 3 shows the global distribution of downward solar radiation atthe surface. We computed a decadal mean global map of solar energy for the period 1949–1958 andfor the period 1999–2008, and then determined the difference between the two. It can be seen thatsolar energy at the surface over the United States decreases. It may imply that climate change may affectthe downward solar radiation at the surface. Decisions about renewable energy developments need toconsider such climate change scenarios.

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AcknowledgmentsThe Author thanks the National Cen-ters for Environment Prediction of theNational Oceanic and AtmosphericAdministration to provide downwardsolar radiation fluxes and meteorolog-ical data for calculating wind power.The author appreciates the great sup-port from Google map.

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