Solar Freaking Roadways, Solar Freaking Disaster!

Solar Roadways is a proposal currently gaining public attention and financing through the crowd funding platform Indiegogo.  The proposal sees the replacement of all roadways, car parks and walkways with electricity producing solar panels.  These solar panels have a number of additional features such as LED road markings, pressure sensitive alerts, heated pads for snow melting and the ability to charge electric vehicles.  This has resulted in a naive speculation of a science fiction like reality.  This short paper uses publicly available data to determine the cost per panel that must be achieved for this product to meet its core objectives i.e. to pay for itself over the life of the product through electricity generation.  The analysis looks at optimised conditions and ignores many of the proposed secondary features.   The question posed is, under what conditions would a single solar roadway panel generate a Net present value of $0?  Assuming technological developments will resolve many of the current issues, this assessment makes assumptions of unreasonably perfect circumstances, namely;

• All cells have an indefinite life expectancy
• No efficiency will be lost in transmission of electricity
• No dust, rubber or inefficiency gained by wear is taken into account.  
• No energy is expelled by the cells, meaning no LED lights are turned on at any time, no warming of tiles for snow covered roads etc
• Cost of energy was obtained on¹ and represents the retail cost per Kw hr
• Panels are calculated at 0.5*0.5 metres with cell coverage of 100% of the surface area. 
• It is assumed that there is no increase in the costs maintenance over conventional blacktop
• The installation costs of solar roadways are assumed to be equivalent to that of blacktop.  

At zenith, the sun deposits 1300 watts of power on the earth.  With the rising and setting of the sun and taking into consideration seasonal changes in its path, the average radiation across the earth at any particular time equals 340 watts per mtr sq 7  (this can be improved to 650 watts by directing a panel towards the sun throughout the day).  Current efficiency of solar cells stands at around 15% and is quoted by the founder of solar roadways as the basis for their calculations.  This results in .01275 Kw per hour of power for each cell.  The current retail cost of power averages across the United States at 12.26 cents per kw hr.  This results in an annual electricity generation of $13.69 per cell per year.   To establish the value of this in perpetuity the annual cash flow is dividend by the expected rate of return.  The current 3 year rolling average market yield for the S&P 500 is 15%² meaning that the expected return must be discounted at this to account for opportunity costs.   With annual cash flows of 13.69 and a discount rate of 15% this generates a present value of all the cost savings for the life of a cell is $91.29.  This means that if a solar roadway panel retails for more than $91, the costs will never be recovered.

Sun energy landing on cell at 12 on the equator (watts)	1300
the average radiation across all of the Earth (watts)	340
Cell size in mtrs sqd	0.25
% efficiency of solar cells	0.15
% coverage with solar cells	1
loss due to glass cover	0
loss due to dust cover	0
loss due to cloud cover	0
Energy expelled for LEDs	0
energy produced per cell	12.75
Kw hrs energy	0.01275
% lost due to transmission	0
Cost of energy in cents / Kw hr in dollars	0.1226
Total cost savings per year $ per cell	13.693194
expected life of a cell	indefinite
Market rate of return	15%
Present value of cost savings ($) for entire life of 1 cell	91.28796

The above scenario represents an idealistic approach and assumes no power consumption is present.  The following scenario represents an optimal real world scenario where cells also light up road markings. The additional assumptions made in scenario 2 are listed below;

• Glass coverage over the solar cells is as efficient as window glass ³
• 69% coverage of each panel with solar cells, as presented by solar roadways
• 20% reduction in efficiency due to dust, tyre wear and scratching ⁴
• 9% of available daylight is under cloud cover resulting in a reduction to 50% efficiency ⁵
• Optimal power transmission conditions are assumed ⁶
• On average, 3 LEDs each at 10 watts are permanently on.
• No panel heating is present
• No additional electronic power is required over and above LED emissions
• Panels have a 25 year life

Sun energy landing on cell at 12 on the equator (watts)	1300
the average radiation across all of the Earth (watts)	340
Cell size in mtrs sqd	0.25
% efficiency of solar cells	0.15
% coverage with solar cells	0.69
% loss due to glass cover	0.04
% loss due to dust cover	0.2
% loss due to cloud cover	0.045
% lost due to transmission	0.042
total energy output (watts)	6.181435987
Energy expelled for LEDs (3 10 watt bulbs)	30
energy produced per cell	-23.5475616
Kw hrs energy	-0.023547562
Cost of energy in cents / Kw hr in dollars	0.1226
Total cost savings per year $ per cell	-25.28951602
expected life of a cell	25
Market rate of return	15%
Present value of cost savings  for entire life of 1 cell	-163.4752018

   

Using a similar process as outlined in the first scenario, the power output has reduced to 6.18 watts.  When considering the 30 watts used to power the LEDs, the net power benefit is -$25.29 per year or -$163.48 based on the present value of a 25 year annuity.  This means each panel will result in an additional $163 in electricity charges in current terms, over and above the costs for construction, sales, distribution and installation.

The solar roadways initiative is plagued with fundamental issues such as the suitability of the material as a road surface, the increased cost of road construction and maintenance, use of LEDs as road markers during the day etc.  However, feasibility of the product can be assessed without a detailed analysis of all subsystems and applications.  If the solar panels are unable to pay for themselves or supply the electricity required to mark the roads then it is fundamentally flawed.  Placing solar cells on a road which is unable to be directed at the sun or placed in a location suitable for solar energy production takes solar cells out of viability.  This is combined with the need to utilise more energy than it is capable of producing and an unnecessarily complex design, the product in question far exceeds the bounds of any cost benefit model.  The core limitations of this product are not design or technical, they are fundamental to the chosen subject matter and are unable to be mitigated.   The goal of this program appears to be concentrated on green energy and cost savings however, it appears that if implemented the net result will be greater use of fossil.  It is reasonable to assume that the founders of solar roadway are fully aware of the limitations of their product.  Test of the initial prototype would have demonstrated the output per panel and the expenditure of electricity to power the lighting.  This is likely the reason behind the lack of technical specifications publicly available.  One can only speculate on the motives of the solar roadway founders, there is no reason to suggest that this is an intentionally fraudulent activity, more likely it is a case of vested interests and cognitive dissonance.  Regardless, further public support is a net economic loss and should not continue.