New Climate Change Related EC Directives and Their Interrelations to Green Electricity and Certificate Practices

Ty��n p����tavoite on tutkia vihre��n s��hk��n ja sertifikaattien kaupan ja EY:n uusien ilmastonmuutosta koskevien direktiivien ja direktiiviehdotusten v��lisi�� yhteyksi��. Tutkimuksessa k��sitell����n direktiivi�� s��hk��ntuotannosta uusiutuvilla energial��hteill�� ja direktiiviehdotuksia Euroopan Unionin alueen p����st��kaupasta sek�� yhdistetyn s��hk��n ja l��mm��n tuotannon lis����misest��. Ty�� keskittyy er����n suomalaisen mets��teollisuusyrityksen toimiin ilmastonmuutoksen hidastamiseksi. Tutkimus keskittyy p����osin EU:n suunnitelmaan aloittaa Unionin j��senvaltioiden v��linen p����st��kauppa, koska t��m�� j��rjestelm�� tulee toteutuessaan olemaan teollisuuden kannalta merkitt��v��. Tilannetta on analysoitu nelj��n sellu- ja paperitehtaan hiilidioksidip����st��laskelmien avulla. Ty��ss�� kehitettyj�� laskumalleja voidaan k��ytt���� avuksi yhti��n muilla tehtailla. T��m��n lis��ksi ty��ss�� on luotu malli energiainvestointien arvioimiseksi tulevaisuudessa ottamalla p����st��oikeuden hinnan vaikutus huomioon. P����st��kaupan vaikutukset pohjoismaisilla vapautuneilla s��hk��markkinoilla on analysoity, koska teollinen s��hk��nhankinta on suuresti riippuvainen t��st�� markkinasta. Suomen mets��teollisuuden oma yhdistetty s��hk��n ja l��mm��n tuotanto erityisesti uusiutuvista energial��hteist�� tulee olemaan entist��kin t��rke��mp���� tiukentuvassa toimintaymp��rist��ss��. T��ll�� hetkell�� on k��ynniss�� kokeilu lis��arvon saamiseksi omalle s��hk��ntuotannolle. T��ll�� haetaan kokemuksia ja valmiutta tulevaa p����st��kauppaa varten.

Adsorption energy and spin state of first-row transition metals adsorbed on MgO(100)

Slab and cluster model spin-polarized calculations have been carried out to study various properties of isolated first-row transition metal atoms adsorbed on the anionic sites of the regular MgO(100) surface. The calculated adsorption energies follow the trend of the metal cohesive energies, indicating that the changes in the metal-support and metal-metal interactions along the series are dominated by atomic properties. In all cases, except for Ni at the generalized gradient approximation level, the number of unpaired electron is maintained as in the isolated metal atom. The energy required to change the atomic state from high to low spin has been computed using the PW91 and B3LYP density-functional-theory-based methods. PW91 fails to predict the proper ground state of V and Ni, but the results for the isolated and adsorbed atom are consistent within the method. B3LYP properly predicts the ground state of all first-row transition atom the high- to low-spin transition considered is comparable to experiment. In all cases, the interaction with the surface results in a reduced high- to low-spin transition energy.

IEA Bioenergy Task 40 ���Sustainable International Bioenergy Trade: Securing supply and demand��� Country report of Finland 2008

This study considered the current situation of biofuels markets in Finland. The fact that industry consumes more than half of the total primary energy, widely applied combined heat and power production and a high share of solid biomass fuels in the total energy consumption are specific to the Finnish energy system. Wood is the most important source of bioenergy in Finland, representing 21% of the total energy consumption in 2006. Almost 80% of the wood-based energy is recovered from industrial by-products and residues. Finland has commitment itself to maintaining its greenhouse gas emissions at the 1990 level, at the highest, during the period 2008���2012. The energy and climate policy carried out in recent years has been based on the National Energy and Climate introduced in 2005. The Finnish energy policy aims to achieve the target, and a variety of measures are taken to promote the use of renewable energy sources and especially wood fuels. In 2007, the government started to prepare a new long-term (up to the year 2050) climate and energy strategy that will meet EU���s new targets for the reduction of green house gas emissions and the promotion of renewable energy sources. The new strategy will be introduced during 2008. The international biofuels trade has a substantial importance for the utilisation of bioenergy in Finland. In 2006, the total international trading of solid and liquid biofuels was approximately 64 PJ of which import was 61 PJ. Most of the import is indirect and takes place within the forest industry���s raw wood imports. In 2006, as much as 24% of wood energy was based on foreignorigin wood. Wood pellets and tall oil form the majority of export streams of biofuels. The indirect import of wood fuels increased almost 10% in 2004���2006, while the direct trade of solid and liquid biofuels has been almost constant.

Pientalojen ilmal��mp��pumput s��hk��verkossa

Ty��ss�� tutkitaan ilmal��mp��pumppujen kokonaisvaltaista vaikutusta s��hk��verkkoon. Tarkastelu aloitetaan l��mp��pumppujen toiminnasta ja rakenteesta, josta jatketaan laitteen k��ytett��vyyteen ja muiden l��mmitysmenetelmien vertailuun. S��hk��isten ominaisuuksien tarkastelussa pohditaan ilmal��mp��pumppujen vaikutusta suomalaiseen s��hk��verkkoon muun muassa yleiss��hk��tekniikan, taloudellisuuden ja energiatehokkuuden sek�� h��iri��iden kannalta. T��m�� tutkielma rajoittuu pientaloihin, ja niihin asennettuihin ilma-ilmal��mp��pumppuihin. Ty��n loppup����telm��n�� on, ett�� ilmal��mp��pumppujen k��yt��st�� ei juuri aiheudu vaikutuksia suomalaiseen s��hk��verkkoon. Suurimmat ilmal��mp��pumppujen k��yt��st�� syntyv��t seuraukset kohdistuvat s��hk��verkkoyhti��ihin, joihin ilmal��mp��pumput aiheuttavat taloudellisia menetyksi��. Suuret ja tulevaisuudessa kasvavat ilmal��mp��pumppum����r��t aiheuttavat s��hk��ntuotantoon lis��tehontarvetta huippukuorman aikaan. Toisaalta v��litehoalueella tehontarve sek�� energiankulutus pienenev��t. S��hk��verkoissa ei ole toistaiseksi havaittu ilmal��mp��pumpuista johtuvia h��iri��it��.

Market of biomass fuels in Finland : IEA Bioenergy Task 40 and EUBIONET III - Country report of Finland 2009

This study considered the current situation of solid and liquid biomass fuels in Finland. The fact that industry consumes more than half of the total primary energy, widely applied combined heat and power production and a high share of solid biomass fuels in the total energy consumption are specific to the Finnish energy system. Wood is the most important source of bioenergy in Finland, representing 20% of the total energy consumption in 2007. Almost 80% of the woodbased energy is recovered from industrial by-products and residues. As a member of the European Union, Finland has committed itself to the Union���s climate and energy targets, such as reducing its overall emissions of green house gases to at least 20% below 1990 levels by 2020, and increasing the share of renewable energy in the gross final consumption. The renewable energy target approved for Finland is 38%. The present National Climate and Energy Strategy was introduced in November 2008. The strategy covers climate and energy policy measures up to 2020, and in brief thereafter, up to 2050. In recent years, the actual emissions have exceeded the Kyoto commitment and the trend of emissions is on the increase. In 2007, the share of renewable energy in the gross final energy consumption was approximately 25% (360 PJ). Without new energy policy measures, the final consumption of renewable energy would increase to 380 PJ, which would be approximately only 31% of the final energy consumption. In addition, green house gas emissions would exceed the 1990 levels by 20%. Meeting the targets will need the adoption of more active energy policy measures in coming years. The international trade of biomass fuels has a substantial importance for the utilisation of bioenergy in Finland. In 2007, the total international trading of solid and liquid biomass fuels was approximately 77 PJ, of which import was 62 PJ. Most of the import is indirect and takes place within the forest industry���s raw wood imports. In 2007, as much as 21% of wood energy was based on foreign-origin wood. Wood pellets and tall oil form the majority of export streams of biomass fuels. The indirect import of wood fuels peaked in 2006 to 61 PJ. The foreseeable decline in raw wood import to Finland will decrease the indirect import of wood fuels. In 2004��� 2007, the direct trade of solid and liquid biomass fuels has been on a moderate growth path. In 2007, the import of palm oil and export of bio-diesel emerged, as a large, 170 000 t/yr biodiesel plant came into operation in Porvoo.

Desenvolvimento de conjunto membrana-eletrodos para c��lula a combust��vel de metanol direto passiva

Direct methanol fuel cells (DMFCs) without external pumps or other ancillary devices for fuel and oxidant supply are known as passive DMFCs and are potential candidates to replace lithium-ion batteries in powering portable electronic devices. This paper presents the results obtained from a membrane electrode assembly (MEA) specifically designed for passive DMFCs. Appropriated electrocatalysts were prepared and the effect of their loadings was investigated. Two types of gas diffusion layers (GDL) were also tested. The influence of the methanol concentration was analyzed in each case. The best MEA performance presented a maximum power density of 11.94 mW cm-2.

ZigBee-verkot ja evaluointilaitteen suunnittelu

In this thesis a closer look at ZigBee wireless network protocol was made and a ZigBee evaluation device was designed and built for testing it. ZigBee is a digital wireless network technology published in 2004, which was developed for wireless control and automation systems. It is based on the IEEE 802.15.4-2003 standard, and its aim was to create a reliable wireless networking protocol with secure data transmissions, low power consumption and low manufacturing costs. The low power consumption requirement has also led to a slow data rate and a short range, but in a ZigBee network individual network devices can route messages to each other, so short range is not a problem. In automation and control systems a fast data rate is not usually an important requirement either. ZigBee network can form automatically and one ZigBee network can theoretically include thousands of devices, all of which can communicate with each other. ZigBee network is also self-healing, so an individual device dropping out of the network, like breaking down, does not affect functionality of the network. Evaluation device was designed and implemented in four different versions, which were tested with six different antenna designs. All antennas were tested at a range of 100 meters in an open terrain using two different frequencies and transmission powers. Also a range test was performed for selected antennas using transmission power of 4,5 dBm, which was maximum power of the ZigBee chip used in the evaluation device. Range test resulted in a measured range of several hundred meters, which can be considered as a good result for a short-range technology.

S��hk��auto osana ��lyk��st�� s��hk��verkkoa ��� Ohjaus ja toiminnallisuudet

Tulevaisuudessa s��hk��verkko kohtaa monia haasteita, kun s��hk��autot yleistyv��t, vaatien suuren tehotarpeen. Uusiutuvan energiantuotannon ep��varma huipputehon tuotanto ei v��ltt��m��tt�� pysty kattamaan s��hk��autoista johtuvaa suurta tehopiikki��, jos suuret m����r��t ajoneuvoista kytket����n yht�� aikaa lataukseen. Jos s��hk��ajoneuvot voidaan ladata ohjatusti, ei v��ltt��m��tt�� tarvita lis��energian tuotantoa kattamaan kasvanutta huipputehon tarvetta. Lis��ksi s��hk��ajoneuvojen akut toimivat koko s��hk��verkolle energiavarastoina, jollaista ei ole ennen ollut. ��lykk����ll�� s��hk��verkolla voidaan ohjata s��hk��ajoneuvon latausta, mik��li ajoneuvossa on ohjausj��rjestelm�� ja akkujen varaustilan mittaus. T��ss�� kandidaatin ty��ss�� ohjelmoidaan mittaus- ja ohjauskortti plug-in hybridiautoa varten, jossa on V2G-ominaisuus. Ohjainkortista toteutetaan toimintakuvaus, jonka mukaan se my��s ohjelmoidaan. Ohjainkortti mittaa akkujen j��nnitett�� ja virtaa, joista voidaan m����ritt���� akkujen varaustilat. Ohjainkortti l��hett���� tiedot eteenp��in PC:lle, jolta ohjainkortti saa k��skyn toimintatilasta. Mittaustietojen perusteella voidaan seurata mahdollisia vikatilanteita. Kandidaatinty��n aikana ohjainkorttia ei ehditty asentamaan ajoneuvoon, mutta laboratoriotestien mukaan voidaan todeta, ett�� ohjainkortti on ohjelmallisesti toimiva. Mittauksissa selvisi, ett�� ohjainkortin mittaustulot eiv��t olleet tarpeeksi tarkkoja k��ytt��kohteeseen. Todettiin, ett�� ohjainkortti vaatii rakenteellisia muutoksia mittaustuloksien parantamista varten, ennen ohjainkortin k��ytt����nottoa, mutta kandidaatinty��n tavoitteet saavutettiin.

Paperitehtaan s��hk��tasehallinta ja s��hk��n kysynt��jousto energiakustannusten hallinnassa

Paperiteollisuus on energiaintensiivist�� teollisuutta, jossa on tehty pitk����n ty��t�� tuotantoprosessien energiatehokkuuden parantamiseksi. Paperitehtaan energiakustannuksiin voidaan kuitenkin vaikuttaa my��s s��hk��taseen hallinnalla ja s��hk��n kysynt��jouston avulla. Tehtaan seuraavan vuorokauden s��hk��n kulutus pyrit����n ennustamaan mahdollisimman tarkasti, mutta esimerkiksi paperitehtaan h��iri��tilanteissa s��hk��tase poikkeaa ennustetusta. T��ll��in s��hk��tasetta voidaan korjata ensisijaisesti s��hk��n j��lkimarkkinoiden eli Elbas- kaupank��ynnin avulla. Ellei tasetta saada korjattua s��hk��n j��lkimarkkinoilla tase-ero korjataan tases��hk��ll��, jonka hinta muodostuu s����t��s��hk��markkinoilla. Tases��hk��n hinta saattaa poiketa Elspot- markkinahinnasta voimakkaasti, jolloin tase-erosta joko hy��dyt����n tai h��vit����n kustannusmieless��. T��m��n ty��n tarkoituksena on selvitt���� s��hk��tasehallinnan parantamisen ja s��hk��n kysynt��jouston vaikutukset paperitehtaan energiakustannuksiin. Ty��ss�� tutkittiin tehtaan s��hk��-tasehallinnan nykytilannetta ja selvitettiin tase-erojen kustannusvaikuttavuutta. Lis��ksi ty��ss�� luotiin ajomalleja s��hk��n kysynt��jouston toteuttamiselle massatehtaan eri tuotantoprosesseille, sek�� m����ritettiin rajahintoja seuraavan vuorokauden energiaennusteeseen. Onnistunut energiaennuste perustuu paperitehtaan k��ynnin tarkkoihin ja ajankohtaisiin l��ht��tietoihin. S��hk��taseen poikkeamiin voidaan puolestaan varautua paremmin, kun informaatio tehtaan prosessien alasajosta tulee voimalaitoksen tietoon mahdollisimman aikaisin. S��hk��taseen poikkeamien hallinta voidaan tehd��, joko Elbas- kaupan tai tases��hk��n avulla. Ajankohdasta ja tasepoikkeaman volyymista riippuen t��ytyy tehd�� valinta, kumpi vaihtoehdoista on kustannusmieless�� kannattavampi. Paperitehtaan eri prosesseille luoduilla ajomalleilla saatiin esiin huomattava s����st��potentiaali. Ajomallien noudattaminen vaatii suunnitelmallista tuotannon hallintaa ja s��hk��n Elspot- hinnan k��ytt��ytymisen s����nn��llist�� seurantaa. Seuraavan vuorokauden rajahintatietojen m����ritt��misen pohjalta voidaan arvioida, mill�� Elspot- hinnalla s��hk��n myynti muuttuu paperin tuotantoa kannattavammaksi.

Design and testing of an armature-reaction-compensated permanent magnet synchronous generator for island operation

At present, permanent magnet synchronous generators (PMSGs) are of great interest. Since they do not have electrical excitation losses, the highly efficient, lightweight and compact PMSGs equipped with damper windings work perfectly when connected to a network. However, in island operation, the generator (or parallel generators) alone is responsible for the building up of the network and maintaining its voltage and reactive power level. Thus, in island operation, a PMSG faces very tight constraints, which are difficult to meet, because the flux produced by the permanent magnets (PMs) is constant and the voltage of the generator cannot be controlled. Traditional electrically excited synchronous generators (EESGs) can easily meet these constraints, because the field winding current is controllable. The main drawback of the conventional EESG is the relatively high excitation loss. This doctoral thesis presents a study of an alternative solution termed as a hybrid excitation synchronous generator (HESG). HESGs are a special class of electrical machines, where the total rotor current linkage is produced by the simultaneous action of two different excitation sources: the electrical and permanent magnet (PM) excitation. An overview of the existing HESGs is given. Several HESGs are introduced and compared with the conventional EESG from technical and economic points of view. In the study, the armature-reaction-compensated permanent magnet synchronous generator with alternated current linkages (ARC-PMSG with ACL) showed a better performance than the other options. Therefore, this machine type is studied in more detail. An electromagnetic design and a thermal analysis are presented. To verify the operation principle and the electromagnetic design, a down-sized prototype of 69 kVA apparent power was built. The experimental results are demonstrated and compared with the predicted ones. A prerequisite for an ARC-PMSG with ACL is an even number of pole pairs (p = 2, 4, 6, ���) in the machine. Naturally, the HESG technology is not limited to even-pole-pair machines. However, the analysis of machines with p = 3, 5, 7, ��� becomes more complicated, especially if analytical tools are used, and is outside the scope of this thesis. The contribution of this study is to propose a solution where an ARC-PMSG replaces an EESG in electrical power generation while meeting all the requirements set for generators given for instance by ship classification societies, particularly as regards island operation. The maximum power level when applying the technology studied here is mainly limited by the economy of the machine. The larger the machine is, the smaller is the efficiency benefit. However, it seems that machines up to ten megawatts of power could benefit from the technology. However, in low-power applications, for instance in the 500 kW range, the efficiency increase can be significant.

Radiaalikompressorin aerodynaaminen suunnittelu

T��m��n diplomity��n tavoitteena on ollut suunnitella radiaalikompressori. Aluksi on tutustuttu radiaalikompressorissa tapahtuviin ilmi��ihin, jonka j��lkeen radiaalikompressori on suunniteltu. Reunaehtoina suunnittelussa olivat toimilaitteelta saatava teho 250 kW ja sen suurin py��rimisnopeus 500 Hz. Esisuunnittelu on tehty Virtaustekniikan laboratoriossa kehitetyll�� CentriFlow-ohjelmalla. Juoksupy��r��n muoto on suunniteltu viskoosittomilla 2D-malleilla. Juoksupy��r��n muodon suunniittelussa on k��ytetty kaupallista AxCent-ohjelmaa. Juoksupy��r��n muoto on tarkistettu laskennallisen virtausdynamiikan avulla. Virtausmallinnuksessa k��ytettiin FinFlo-ohjelmaa. Suunnittelun ja mallinnuksen pohjalta valittiin kolme erilaista juoksupy��r���� valmistukseen.

Review of water electrolysis technologies and design of renewable hydrogen production systems

An electric system based on renewable energy faces challenges concerning the storage and utilization of energy due to the intermittent and seasonal nature of renewable energy sources. Wind and solar photovoltaic power productions are variable and difficult to predict, and thus electricity storage will be needed in the case of basic power production. Hydrogen���s energetic potential lies in its ability and versatility to store chemical energy, to serve as an energy carrier and as feedstock for various industries. Hydrogen is also used e.g. in the production of biofuels. The amount of energy produced during hydrogen combustion is higher than any other fuel���s on a mass basis with a higher-heating-value of 39.4 kWh/kg. However, even though hydrogen is the most abundant element in the universe, on Earth most hydrogen exists in molecular forms such as water. Therefore, hydrogen must be produced and there are various methods to do so. Today, the majority hydrogen comes from fossil fuels, mainly from steam methane reforming, and only about 4 % of global hydrogen comes from water electrolysis. Combination of electrolytic production of hydrogen from water and supply of renewable energy is attracting more interest due to the sustainability and the increased flexibility of the resulting energy system. The preferred option for intermittent hydrogen storage is pressurization in tanks since at ambient conditions the volumetric energy density of hydrogen is low, and pressurized tanks are efficient and affordable when the cycling rate is high. Pressurized hydrogen enables energy storage in larger capacities compared to battery technologies and additionally the energy can be stored for longer periods of time, on a time scale of months. In this thesis, the thermodynamics and electrochemistry associated with water electrolysis are described. The main water electrolysis technologies are presented with state-of-the-art specifications. Finally, a Power-to-Hydrogen infrastructure design for Lappeenranta University of Technology is presented. Laboratory setup for water electrolysis is specified and factors affecting its commissioning in Finland are presented.

Electrical Safety of Island Operated Low Voltage DC Network

Today, renewable energy technologies and modern power electronics have made it feasible to implement low voltage direct current (LVDC) microgrids (MGs) ca-pable to island operation. Such LVDC networks are particularly useful in remote areas. However, there are still pending issues in island operated LVDC MGs like electrical safety and controlled operation, which should be addressed before wide-scale implementation. This thesis is focused on the overall protection of an island operated LVDC network concept, including protection against electrical shocks, mains equipment protection and protection of photovoltaic (PV) power sources and battery energy storage systems (BESSs). The topic is approached through ex-amination of the safety hazards and the appropriate methods to protect against them, comprising considerations for earthing system selection and realisation of the protection system.

��valuation de nouveaux ��lectrolytes �� base de liquides ioniques protiques en supercapacit��s asym��triques de type MnO2/carbone

Les supercapacit��s hybrides se taillent de plus en plus une place dans le secteur des ��nergies renouvelables. L���oxyde de mangan��se poss��de certaines caract��ristiques (faible co��t, caract��re ��cologique et une capacitance sp��cifique sup��rieure) qui font de ce dernier un mat��riau tr��s attirant pour l���assemblage de tels dispositifs. Ce m��moire, divis�� en trois parties, propose l���utilisation des liquides ioniques protiques comme ��lectrolytes pour l���assemblage de supercapacit��s hybrides �� base d���oxyde de mangan��se et carbone. D���abord, le comportement pseudocapacitif des ��lectrodes �� base de films minces d���oxyde de mangan��se dans les liquides ioniques protiques ainsi que leurs propri��t��s optiques sont ��tudi��s et ��valu��s. Des valeurs de capacitance sp��cifique allant jusqu����� 397 F/g ont ��t�� trouv��es dans cette partie. Ensuite, des m��langes compos��s par un solvant organique (ac��tonitrile) et le liquide ionique protique sont pr��sent��s comme une mani��re de contourner la faible conductivit�� de ce dernier qui limite les valeurs de capacitance sp��cifique �� obtenir. Une am��lioration de la capacitance sp��cifique d���environ 30% est report��e dans ce chapitre. Finalement, l���assemblage d���une supercapacit�� hybride est pr��sent�� comme une strat��gie efficace qui permet l�����largissement de la faible fen��tre de potentiel de travail obtenue avec les ��lectrodes �� base d���oxyde de mangan��se. De cette fa��on, la faisabilit�� de tel arrangement est montr�� ici, obtenant de valeurs de capacitance sp��cifique (16 F/g) ainsi que de puissance (81 W/kg) et d�����nergie sp��cifique (1,9 Wh/kg) acceptables en utilisant des liquides ioniques protiques comme ��lectrolytes de remplissage.

Investigations on polyaniline based systems for technologically important applications and some novel polymers as prospective candidates for fabricating polymer light emitting diodes

Light emitting polymers (LEP) have drawn considerable attention because of their numerous potential applications in the field of optoelectronic devices. Till date, a large number of organic molecules and polymers have been designed and devices fabricated based on these materials. Optoelectronic devices like polymer light emitting diodes (PLED) have attracted wide-spread research attention owing to their superior properties like flexibility, lower operational power, colour tunability and possibility of obtaining large area coatings. PLEDs can be utilized for the fabrication of flat panel displays and as replacements for incandescent lamps. The internal efficiency of the LEDs mainly depends on the electroluminescent efficiency of the emissive polymer such as quantum efficiency, luminance-voltage profile of LED and the balanced injection of electrons and holes. Poly (p-phenylenevinylene) (PPV) and regio-regular polythiophenes are interesting electro-active polymers which exhibit good electrical conductivity, electroluminescent activity and high film-forming properties. A combination of Red, Green and Blue emitting polymers is necessary for the generation of white light which can replace the high energy consuming incandescent lamps. Most of these polymers show very low solubility, stability and poor mechanical properties. Many of these light emitting polymers are based on conjugated extended chains of alternating phenyl and vinyl units. The intra-chain or inter-chain interactions within these polymer chains can change the emitted colour. Therefore an effective way of synthesizing polymers with reduced ��-stacking, high solubility, high thermal stability and high light-emitting efficiency is still a challenge for chemists. New copolymers have to be effectively designed so as to solve these issues. Hence, in the present work, the suitability of a few novel copolymers with very high thermal stability, excellent solubility, intense light emission (blue, cyan and green) and high glass transition temperatures have been investigated to be used as emissive layers for polymer light emitting diodes.