# NREL Releases Updated Baseline of Cost and Performance Data for Electricity Generation Technologies 2016

Every year the cost of power generation projects are changing in function of technological changes, changes in prices of supplies or due to economies of scale.
NREL has recently publish the  2016 version of the Baseline of Cost and Performance of  most technologies used nowadays for power generation showing the flat trend of prices for conventional energy and the impressive decline in prices that have renewable energies, especially Solar PV with Capital Costs of up to 0.9 [MM USD / MW] for PV at utility scale.
I’m excited to think that the day has come when renewable energy is economically competitive with traditional energies and has reached the point where they no longer need subsidies to displace old and polluting forms of energy production.

Clearly the future is greener…

# Solar irradiance on sunny and cloudy days

The solar radiation that is emitted from the sun in the form of electromagnetic waves, entering the atmosphere is filtered by the gases it contains, mainly by clouds that are floating, which bounce the light to operate as mirrors. When passing a cloud, global radiation decreases dramatically and diffuse radiation increases as a result of the deviation in the direction of the aces of light that causes the cloud.

Figure 1: Solar radiation in a sunny day

Figure 2: Solar radiation in a partly cloudy day

# Solar radiation spectrum and spectral response of thermopile

Most of the energy that it is emitted from the Sun is in form of electromagnetic radiation with a specific spectrum given by the temperature of his external layer. The little part of this energy that arrive to earth is our source of life and energy.

The radiation that arrives to the external layer of the earth to a normal plane, before been filtered by the atmosphere is called Extraterrestrial Radiation and can be approximately calculated by :

$\mbox{E}_{ext}\; =\; \mbox{E}_{sc}\; \cdot \; \left( 1+0.033412\cdot \cos \; \left( 2\pi \frac{dn-3}{365} \right) \right)$

been

$\mbox{E}_{sc}$ =  Solar constant, 1367 [watt/m2]
dn = day number of year (1 … 365)

Then the radiation is filtered by the gases presents in the atmosfere like H2O, CO2, O3 and O2, and reflected by the clouds. The radiation that finally arrives to the lands and oceans it’s in part absorbed and part reflected.

The absorbed radiation is transformed in heat and emitted back to the space like infrared radiation.

Image from Kipp & Zonen

To measure the solar radiation that arrives on Earth requires an instrument with a thermopile, if this is design to measure infrared wavelength the instruments it’s call Pyrgeometer but if is design to measure the visible spectrum It’s call Pyranometer. A good thermopile must have a flat response to the whole spectrum that is measuring.

Image from Kipp & Zonen