Renewable wind and solar power are important solutions to reducing carbon emissions.  These technologies have improved substantially in recent years.  Despite very significant advancements, wind and solar power costs continue to be greater than existing low carbon alternatives such as natural gas.  In addition expanding variable wind and solar power requires increasing amounts of peaking power backup capacity to properly control and maintain critical power grids reliabilities.  Why does expansion of wind and solar power continue to require increasing levels of natural gas electric power generation?

Daily Electric Power Demand Varies – When you woke up this morning you probably turned on the lights and might have turned up the heat to take the chill off the room.  You may have prepared a hot breakfast, and either turned on the TV to check the weather and traffic reports, or went on-line to check your emails.  Before you left the house you also might have turned on the dish washer and possibly started the day’s laundry.   Many of these activities are fairly common for most people in preparing themselves for the day’s activities.  While you were preparing yourself for the day, many commercial businesses, stores, offices, public schools, etc. also began opening and powering up their daily operations.

What most people do not think much about is that their electric power demand varies significantly during the day and the power grid operations must be continuously adjusted to meet changing demands.  For example, individual residences power consumption increases from minimum levels to maximum levels as the day progresses.  Refer to the following ‘power load curve’ for typical U.S. residences.

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Average power load curve based on PATH data.

Typical U.S. Residential power consumption increases from about 0.5 KWH up to 5 KWH during the day.  During the late night-early morning your refrigerator/freezer, HVAC system, and charging laptop/tablets or smart-phones consume minimum power levels for a given day.  After you wake-up and begin turning on various lights, appliances, electronics and adjust room temperatures your power consumed rapidly increases to many times the minimum levels that occurred while you slept.

The Commercial sector has a similar daily average power demand load curve pattern compared to the Residential sector.  The Industrial sector’s daily power demand is relative constant.  As a result, total power demand varies significantly during a given day.

Electric Power Supply Changes with Demand – Power grids are normally designed to continuously adjust the level of power supply with varying demands.  The majority of Residential and Commercial customers are supplied ‘on-demand’ power.  This means that customers can increase or decrease their power demand most any time.  The operators of power grids must rapidly increase or decrease power generation supplies as required to meet daily demand changes and balance power supply-demand.  This is accomplished by rapidly increasing or decreasing intermediate or ‘peaking’ load natural gas power generation.  Most stable or baseline-minimum power demand is supplied by ‘baseload’ power generation.  To illustrate refer to the following supply and demand load curves.

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Power load curves base on EIA MERNew England, and related studies data

As shown, total power demand varies from high levels during the mid-/late-day, to low levels at night-early morning.  Baseload power nuclear, coal and hydropower (including fully ‘dispatchable’  biomass) are normally operated at relatively constant rates.  Varying power demand is primarily supplied by baseload-intermediate-peaking natural gas power.  Note: due to the relatively small levels of wind+solar power, these renewables are assumed to be constant for this power load curve example.

Depending on the availability of ‘non-dispatchable’ wind and solar power, these variable power generation sources normally reduce the need for peaking natural gas power.   How much and when power is available from wind or solar sources depends on a number of variables.  Solar power cannot become significant until after sunrise and normally does not produce maximum power generation until mid-day.  Wind power generation depends on wind conditions during a given day.  If the renewable power facilities experience calm winds or cloudy conditions, both wind and solar power production will be reduced.

To illustrate how variable wind and solar power can be, Germany’s experience is an excellent example.  In 2012 Germany expanded its wind and solar power to record levels.  Recently published data on actual power generation performance shows how extremely volatile and unpredictable wind and solar can be in the short- and long-terms.  Refer to pages 87-138 of the recently published Fraunhofen Institute report.

Maintaining Power Grid Reliability – Power grid operators are responsible to continuously monitor and maintain the balance between supply-demand and overall grid (voltage) stability.  If supply-demand balances are not controlled within safe operating limits, power grid stabilities and reliability will be automatically adjusted.  Safety control devices (circuit breakers, power generation trips, etc.) begin automatically shutting off uncontrollable demand or shutting down excessive power supplies to protect power grid equipment and transmission systems from overheating and mechanical failures.  These auto-safety control actions result in black-outs and brown-outs for all customers on the affected power grid(s).

To reliably and safely control power supply without forcing demand changes, advanced control systems have been installed to maintain critical power grid supply-demand balances and stabilities.  What some people refer to as ‘smart grid’ technologies actual begins with macro system controls designed to react and make adjustments to short-term changes in power demand (feed-back controls).  These systems also include advanced computer system-controls that anticipate normal daily changes in power demand (feed-forward controls).  The combination of automatic control systems (and operator manual adjustments when needed) ensure that power generation capacity is continuous adjusted to maintain power grid dynamic, real-time stabilities as required for continuous efficient, reliable and safe operations. 

Wind and Solar Power Impacts on Power Grid Stabilities – Power grid supply-demand balances are normally controlled by adjusting the level of power generation supply in response to demand changes.  With the exception of a few industrial or public utility ‘interruptible’ customers and available hydropower pumped storage, most power grids can only maintain supply-demand balances by adjusting power generation supply.  Although wind and solar are technically power generation sources, these variable-unpredictable supply sources add significantly to the volatility and difficulty in properly controlling power grid supply-demand balances.

Unlike most power generation facilities that are fully dispatchable (can be scheduled, started up, shutdown and adjusted as demand requires), non-dispatchable or variable wind and solar power cannot be scheduled or readily adjusted as system demand requires.  The level of wind and solar power generation is conditional upon the weather and time of day.  While solar power is generally more predictable than wind power, it obviously cannot operate at night and wind can (part-time).  These performance differences have obvious advantages towards displacing fossil fuels, but also have the major disadvantage of making the control of power grid supply-demand much more difficult (depending on the percentage or level of ‘penetration’ into a given grid’s total power mix-supply). 

Due to the unpredictable or non-dispatchable nature of wind and solar, these power generation sources are commonly referred to as ‘negative demands’.  Since heavy cloud cover and too high/low winds cannot be predicted with a high level of certainty, wind and solar power must be fully backed up with peaking power supply capacity such as natural gas.  Similar to adjusting and balancing power grid’s supply-demand when ‘on-demand’ customers significantly change their power usage (without notification or constraints), the loss or gain of wind and solar power must be similarly controlled by adjusting (natural gas) peaking power generation capacities.

Expanded Wind and Solar Power Requires Increasing Levels of Natural Gas Peaking Power – Coal and nuclear power is normally only available for relatively constant baseload power capacity, which is planned and scheduled to minimize rate change frequencies and magnitudes.  Besides maximizing baseload power generation efficiency, coal power operating flexibility is further limited by the need to strictly control plant stack emissions.  Due to these operating constraints coal and nuclear power plants are not suitable sources as peaking or backup power to variable wind and solar power. 

Wind and solar power have average capacity factors of 33% and 20-25% respectively.  This means during a given period of time (day, week, etc.), renewable wind/solar is only capable of supplying full design power generation capacity to the grid on-average about 20-33% of the time.  Since wind and solar are variable and unpredictable, peaking power must be on-line 100% of the time.  Peaking power must be on-line at some minimum rate and available to quickly adjust to variable renewables power supply changes as required to continuously control power grids supply-demand balances within operating safety limits.

Natural gas is an excellent source of both peaking and baseload electric power supply.  Due to its high capacity factor (87%), high efficiency and relatively low fuel cost and emissions, natural gas power supplies power grids reliably and cost effectively compared to other currently available peaking power alternatives (petroleum, biogas, etc.).  These factors make natural gas peaking power the ideal backup for increasing penetration levels of wind and solar power supply.  Since variable wind and solar power cannot be used to displace constant-baseload power such as coal, these variable power sources are only capable of displacing natural gas peaking power capacity and associated fuel consumption.

Power Storage and Interruptible Demand Options – Current options to either storing electric power or reducing demand are relatively limited.  Hydropower pumped storage is the only industrial available  option for reasonably and efficiently storing and supplying on-demand power to connected grids.  Another available option is ‘interruptable’ Industrial and Public utilities customers.  Some Industrial customers can reduce their power consumption significantly on short notice by either reducing operations (throughput-production) or switching to backup (onsite) power.

Some Public utilities such as waste or fresh water treatment plants are also built to operate with interruptable power supply contracts.  This capability is achieved by building larger capacity water treatment facilities that can meet total customer demand by operating part-time at higher rates, and installing storage for receiving waste or supplying fresh water to customers during periods of power interruption.  Interruptable customers, of course, are normally compensated with lower power costs than non-interruptable customers.

Although hydropower pumped storage is the only industrial available power storage option available today, future develops are possible and are definitely needed for significantly expanding variable wind and solar power penetrations into existing power grids.  Possible power storage options such as various thermal or chemical energy conversion, capacitor/battery, static potential energy, compressed air, dynamic mechanical, etc. must be developed.  New future energy storage systems, however, must reasonably compete with or exceed the energy efficiency and costs of proven hydropower pumped storage technology.

Adjustable Wind and Solar Power Generation – While wind and solar power cannot be increased once maximum generation is achieved with available wind/sun, these renewable supplies can be reduced and adjusted to lower power generation levels to help stabilize local power grid supply-demand balances.  State-of-art wind turbine blade pitches can be readily adjusted to reduce power outputs and solar PV panel arrays can be adjusted to reduce power generation.  Most countries, however, put priority on maximizing renewable wind and solar power generation into connected grids.

In the U.S. the level of wind and solar power penetrations is relatively small.  Refer to the following table.

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EIA MER data.  Note: Almost 90% of dispatchable renewable power generation is supplied by hydropower. 

Even though the level of U.S. wind+solar power has increased by 640% since 2005, today these renewables still only account for 3.5% of total net power generation.  Baseload coal power has decreased from 51% in 2005 to 38% today.  Nearly all of this reduced coal power generation has been replaced by natural gas.  Variable wind+solar power have reduced the need for total natural gas (peaking) power by about 10% 2012.

Germany, the world’s leader in wind and solar power, has increased these variable power sources to levels that are causing increasing regional power grid reliability issues.  Rather than building or ensuring adequate local peaking power is available to maintain in-country power grid stability, Germany has the advantage (or has taken advantage) of their neighbors who are integrated into regional EU power grids.  Rather than adjusting peaking power within Germany, the Germans are exporting their excess, variable power to adjacent countries.  This forces Germany’s neighbor countries to reduce their peak, intermediate and baseload power generation.  Although these variable, unscheduled exports are generally delivered at below market average prices, the lower costs do not necessarily take into account the full impacts of uncontrollable ‘negative demand’ impact levels on overall EU regional power grids performances.

In conclusion – Renewable wind and solar power are clearly among the strongest options to replacing fossil fuels power generation.  The penetration of these variable power generation technologies is constrained by costs and the available backup peaking power sources such as natural gas.   Until reliable backup-peaking power options including adequate industrial scale power storage is developed or substantially increased levels of interruptable power demand is made available, up to 100% backup power from reliable sources such as natural gas peaking plants will continue be required to support significant levels of variable wind and solar power in the future.  Required natural gas peaking power backup will continue to increase proportionally to expanded wind and solar power capacity until cost effective alternatives are developed.