Im­pact of high RE ad­di­tion on ther­mal plants

Power Watch India - - COVER STORY TETE-A-TETE - By M Ra­jagopalan

When a sub­stan­tial amount of re­new­able en­ergy (RE) based in­stal­la­tion is be­ing planned, a per­ti­nent ques­tion to ask is: “What ca­pac­ity (MW) of con­ven­tional power plants will the ad­di­tional RE in­stal­la­tion help avoid in­vest­ments in or, per­haps even, help in re­tir­ing some of the ex­ist­ing coal MW?

To ex­plain, Tamil Nadu has an in­stalled ca­pac­ity of over 8000 MW of wind tur­bines. What is the ca­pac­ity credit or com­fort that this in­stalled ca­pac­ity of­fers? In­vest­ments in how many MW of coal plant ca­pac­ity could be avoided? The hon­est an­swer is: “Zero MW”. That’s be­cause there are many 15-minute time blocks in the year in which wind doesn’t blow at all. Des­patch­able ca­pac­ity from con­ven­tional plants such as coal must not only be avail­able in the form of in­stalled ca­pac­ity, but must be kept run­ning (“spin­ning”), to back up the short­fall.

There are many who of­fer a com­fort­ing ex­pla­na­tion that wider ge­o­graphic dis­tri­bu­tion of RE and im­proved trans­mis­sion through green cor­ri­dor, etc. will smoothen out the vari­abil­ity of RE, tak­ing ad­van­tage of vary­ing de­mands in dif­fer­ent parts of the coun­try. This is not backed by any ev­i­dence, as mod­els show that peak de­mand across dif­fer­ent re­gions tend to over­lap and ex­cess RE is rarely avail­able. Other so­lu­tions such as “En­ergy stor­age” etc, are yet to pass the tests of scal­a­bil­ity and af­ford­abil­ity.

So, if In­dia is plan­ning an RE ad­di­tion of 175 GW, it prac­ti­cally means that an equiv­a­lent ca­pac­ity of des­patch­able gen­er­a­tion must be kept avail­able in hot readi­ness. So­lar PV will not con­trib­ute a sin­gle MW dur­ing night time when the de­mand peaks, and coal plants must be kept run­ning on low load dur­ing day­time to al­low ab­sorp­tion of so­lar gen­er­a­tion and to be ready for even­ing ramp-up.

So, in a sit­u­a­tion where 200 GW of RE ca­pac­ity and 230 GW of ther­mal ca­pac­ity to­gether need to meet a peak de­mand of 180 GW, (and an av­er­age de­mand of 150 GW), and if it is man­dated that en­tire RE gen­er­a­tion must be ab­sorbed pref­er­en­tially, the only way to do this is by cy­cling down coal plants- to a low PLF- some­times even bring­ing the load down to a tech­ni­cal min­i­mum. If tech­ni­cal min­i­mum is reached, the grid op­er­a­tor will have no option but to cur­tail RE.

So, the option avail­able is to run coal plants at low PLF with con­se­quences of a) Higher fuel con­sump­tion per kWH, due to lower ef­fi­ciency, and higher CO2 emis­sion per kWh b) Higher fixed cost per kWh due to poorer amor­ti­sa­tion and c) Higher main­te­nance costs due to fre­quent cy­cling of the plants and us­ing liq­uid fuel at low loads. It can be shown that when a coal plant op­er­ates at 55% load in­stead of nor­ma­tive 85%, the add-on costs can be nearly Rs 1/kWh on ac­count of these three fac­tors. The other option is to cur­tail RE gen­er­a­tion, de­feat­ing the pur­pose of adding them in the first place. This is ex­actly what’s hap­pen­ing in China, where 21% of wind gen­er­a­tion was cur­tailed last year, to pro­tect ther­mal plants.

A way out of the sit­u­a­tion is to in­tro­duce suit­able, any­time-des­patch­able so­lu­tions that need not be kept spin­ning, but which can, nev­er­the­less, come on­line in quick time to re­store power lost due to RE short­fall. Hy­dro plants must be pro­moted in states where re­sources ex­ist. In the US, mar­kets in Cal­i­for­nia and Texas are wit­ness­ing adop­tion of IC-en­gine tech­nol­ogy that have the flex­i­bil­ity to start and ram­pup to full load in 2-5 min­utes if wind fails and to with­draw in­stantly when wind comes back. Mo­du­lar de­sign of these plants also en­able despatches in smaller chunks at best ef­fi­ciency.

In­dia should make a course-cor­rec­tion in its ca­pac­ity ad­di­tion, so as to have a proper, ef­fi­cient and flex­i­ble grid with the right mix. At least 20% of planned coal-plants ad­di­tion can be re­placed with flex­i­ble gen­er­a­tion.

The au­thor is Mar­ket Devel­op­ment Di­rec­tor (Mid­dle East & Asia), Wart­sila In­dia Pvt Ltd.

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