SO­LAR FOR DISTRICT COOL­ING

Utilities Middle East - - CONTENTS -

A look at how the on­go­ing de­vel­op­ment of so­lar power in KSA as an al­ter­na­tive source of en­ergy could gain wide adop­tion in district cool­ing in­dus­try

A look at how the on­go­ing de­vel­op­ment of so­lar power in Saudi Ara­bia as an al­ter­na­tive source of en­ergy could gain wide adop­tion in its grow­ing district cool­ing in­dus­try.

Two re­searchers, Guiseppe Fran­chini and An­to­nio Perdichizzi, from the Univer­sity of Berg­amo have pro­posed an al­ter­na­tive cool­ing tech­nol­ogy; us­ing a ther­mal form of so­lar en­ergy to pro­vide so­lar district cool­ing in Saudi Ara­bia.

Their study of the eco­nomics of so­lar district cool­ing was pub­lished at En­ergy Con­ver­sion and Man­age­ment, in Per­for­mance pre­dic­tion of a so­lar district cool­ing sys­tem in Riyadh, Saudi Ara­bia – a case study.

“Our pro­posed so­lu­tion is a so­lar district cool­ing sys­tem for new set­tle­ments in Saudi Ara­bia. This con­cept com­bines so­lar cool­ing and district cool­ing,” said Fran­chini.

The study has im­pli­ca­tions for the cli­mate. With one of the world’s high­est per capita en­ergy con­sump­tion rates, cool­ing ac­counts for more than 70% of elec­tric­ity use in Saudi Ara­bia’s year round hot cli­mate.

But the King­dom still pro­duces this elec­tric­ity al­most en­tirely us­ing fos­sil fu­els, de­spite its abun­dant so­lar re­source.

WIDER AP­PLI­CA­TION IN THE WORLD’S HOT­TER FU­TURE

Us­ing so­lar heat for district cool­ing has seen in­creased in­ter­est as the need for cool­ing is ex­pected to grow as the cli­mate heats up, not only in Saudi Ara­bia, but in many re­gions.

“Cli­mate change is a good chal­lenge be­cause of the in­crease of the cool­ing load ex­pected due to in­creas­ing tem­per­a­tures,” Fran­chini added.

“So this tech­nol­ogy could be in­ter­est­ing also for other lo­ca­tions like in the Mediter­ranean. In Italy for ex­am­ple, cool­ing loads are in­creas­ing more and more with the higher tem­per­a­tures in the sum­mer. So this tech­nol­ogy can have a good ap­pli­ca­tion also in other coun­tries.”

Early in their in­ves­ti­ga­tion, Perdichizzi and Fran­chini de­ter­mined that so­lu­tions based only on PV ex­hibit some weak­ness points: the main crit­i­cal is­sue is re­lated to the elec­tric stor­age by bat­ter­ies for large size sys­tems. So their fo­cus be­came so­lar ther­mal tech­nolo­gies. But which

tech­nol­ogy would be best?

“Our in­ves­ti­ga­tion was com­par­ing so­lar district cool­ing from par­a­bolic trough col­lec­tors or so­lar cool­ing from evac­u­ated tube col­lec­tors,” said Perdichizzi.

“One is based on sin­gle stage ab­sorp­tion chiller driven by evac­u­ated tube col­lec­tors op­er­at­ing at medium tem­per­a­tures. The sec­ond one is based on two stage ab­sorp­tion chillers driven by par­a­bolic troughs op­er­at­ing at a higher tem­per­a­ture around 170°C.”

Since an ab­sorp­tion chiller does not re­quire very high tem­per­a­tures for cool­ing (about 170°C for dou­ble-stage and 100°C for sin­gle-stage), the ob­vi­ous choice would seem to be the typ­i­cal so­lar ther­mal rooftop col­lec­tors, such as is pop­u­larly used to heat swim­ming pools or to make hot wa­ter in homes at com­par­a­tively low tem­per­a­tures.

Yet af­ter com­par­ing the two, they found that a par­a­bolic trough so­lar field would be the most eco­nom­i­cal.

In this case the so­lar col­lec­tors would not be con­nected to a power block, as they would be for gen­er­at­ing elec­tric­ity (nor­mally at about 400°C in trough CSP). In­stead, the curved mir­rors would sim­ply be used to con­cen­trate so­lar en­ergy in or­der to heat wa­ter to just 170°C.

The study as­sumed a pro­posed district of 100 build­ings, with each struc­ture hous­ing up to ten peo­ple.

Us­ing par­a­bolic trough col­lec­tors, the so­lar field needed would oc­cupy about 9,000 sq me­ters: a bit larger than a foot­ball field. But a lot more space would be needed, about 15,000 sq me­ters, to sup­ply the needed ther­mal en­ergy from evac­u­ated tube “rooftop” so­lar col­lec­tors.

REL­A­TIVELY LOW TEM­PER­A­TURE NEED

The so­lar field would sup­ply a two-stage ab­sorp­tion chiller, at less than half the tem­per­a­ture pro­duced by trough CSP for gen­er­at­ing elec­tric­ity in a power block.

“The two-stage ab­sorp­tion chiller is driven by hot wa­ter at just 170°C and pro­duces chilled wa­ter at 5 or 7°C which is used for district cool­ing,” ex­plained Fran­chini.

“Two-stage ab­sorp­tion chillers can be driven by dif­fer­ent sources; by steam, or by pres­sur­ized wa­ter. In our case we sup­pose that it is driven by pres­sur­ized liq­uid wa­ter and heated by the par­a­bolic trough so our scheme is based on the field of par­a­bolic troughs pro­duc­ing a pres­sur­ized hot wa­ter tem­per­a­ture of 170° Cel­sius.”

The re­searchers chose not to cover 100% of the an­nual cool­ing need, be­cause the peak load, 4-5 MW, is typ­i­cally only for a week in sum­mer. An aux­il­iary elec­tric chiller would cover that brief peak.

“Typ­i­cally a so­lar frac­tion of 100% is too high be­cause it means over­siz­ing the so­lar field – yet the ab­sorp­tion chiller would only be used for a few hours per year. So the de­sign is for a cool­ing so­lar frac­tion of about 80%,” Perdichizzi ex­plained.

“The re­main­ing 20% would be cov­ered by an aux­il­iary con­ven­tional chiller us­ing elec­tric­ity.”

The in­su­lated pipe­lines of 12,000 me­ters through­out the district con­sti­tutes cold stor­age in a sense, with a tem­per­a­ture loss of just 1-2 de­grees overnight.

WIDER APPLICATIONS FOR SO­LAR DISTRICT COOL­ING

The re­searchers say so­lar district cool­ing could be ei­ther de­signed into a new district from scratch or retro­fit­ted to ex­ist­ing districts, and district cool­ing could be ap­plied at dif­fer­ent scales.

“This tech­nol­ogy could be ap­plied at dif­fer­ent sizes. For dif­fer­ent com­mu­ni­ties whether small, medium, or large. Of course all this equip­ment should be scaled up in­creas­ing the size,” said Perdichizzi.

“What we an­a­lyzed was a com­pound of per­haps 100 just be­cause that’s a very com­mon size for new com­pounds be­ing built in Saudi Ara­bia.”

They mod­eled two pos­si­ble sites, on the coast at Jed­dah, and in­land at Riyadh. Although ini­tially ap­peal­ing (a source of wa­ter), Jed­dah proved less suit­able, due to the detri­men­tal ef­fect of the high hu­mid­ity lev­els close to the sea, which re­duced the avail­able so­lar re­source.

They used mod­els to pre­dict the per­for­mance of the sys­tem, by the hour, through­out all of the year, to cal­cu­late the ex­act sav­ing of en­ergy and fos­sil fuel and car­bon diox­ide, to cost and demon­strate the tech­ni­cal fea­si­bil­ity of the en­tire sys­tem, in part­ner­ship with King Ab­dul­lah City for Atomic and Re­new­able En­ergy (K.A.KARE).

“Af­ter the cool­ing load cal­cu­la­tion we de­signed a pipe net­work from the cool­ing sta­tion to the build­ings,” said Perdichizzi. “You need to know how much is the en­ergy for de­liv­er­ing wa­ter in the pipes, their di­am­e­ter, their in­su­la­tion. And fi­nally we are able to carry out sim­u­la­tions of the so­lar field and the ab­sorp­tion chiller, and the in­te­gra­tion of the two un­der real me­te­o­ro­log­i­cal data. A full op­ti­miza­tion of the sys­tem is what is new.”

What makes their study unique is that their first aim was to op­ti­mize the en­ergy con­sump­tion of the vil­las served within the district to cre­ate a com­pre­hen­sive district cool­ing plan.

To this end, the two re­searchers lever­age their pre­vi­ous ex­pe­ri­ence in ar­chi­tec­tural en­ergy ef­fi­ciency, as the de­sign­ers of the first Pas­sive Haus in Dubai, which is a fully au­tonomous net zero de­sign of a 700 square me­ter build­ing; tap­ping into that re­lated ex­per­tise in in­ves­ti­gat­ing district so­lar cool­ing.

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