Photovoltaic Systems Cameron Johnstone Department of Mechanical Engineering Room M6:12

Direct conversion of solar radiation into electricity. Semi-conducting material Silicon- Mono crystalline $800/m 2 or $5.5/ W p - Poly crystalline $650/m 2 or $5.4/ W p - Amorphous (thin film) $450/m 2 or $7.5/ W p Output/cm 2 - V oc = 0.6 V - I sc = mA Photovoltaics: Introduction

Silicon responsive to: 0.3  m < < 1.1  m  Absorb 80% solar flux but respond to 55% of the spectral intensity Photovoltaics: Spectral response

Photovoltaics: Performance enhancement Prevent recombination (Barrier creation) Performance enhancement Doping 1 part per Phosphorus -> N type Si Boron -> P type Si Maximise photon absorption Anti reflective coatings Grooving of Si

Photovoltaics: Performance quantification Performance characterisation at Standard Test Conditions (STC) G = 1000 W/m 2 T cell = 25°C Record: I sc V oc I max &V max Maximum Power (P max ) P max = I max * V max Fill Factor (FF) FF =I max * V max I sc * V oc (for C-Si 0.75 – 0.85)

Photovoltaics: Effects of temperature Effects of Temperature V oc  1/T I sc  T V oc = V oc (Tref) [ 1 -  v (T - T ref ) ] I sc = A[I sc (Tref) +  c (T - T ref ) ] where:  v = V oc temperature coefficient (V/  C) A = constant independent of temperature  c = I sc temperature coefficient (A/  C)  V max

Photovoltaics: Effects of temperature Effects of Temperature P out  1/T P max = P max(Tref) [ 1 - P p (T - T ref ) ] where: P p = P max temperature coefficient (%/  C) (for C-Si 0.3 – 0.45) Impact of Irradiance I  G (doubling irradiance (G) doubles I max ) Small ΔV Large Δ I

Photovoltaics: Maximum power point tracking Maximises Efficiency of power delivery from PV: - - Provide load control optimisation by tracking V max for variations in G and T - - Load resistance varied via DC-DC converter (maintains output voltage but limit supply current) - - Integrated within inverter systems - - Unit costs from £400 ΔVΔVΔVΔV

Photovoltaics: Grid integration Converts PV output to grid voltage (DC-AC inverter) - - Sinusoidal Inverter (12V, 24V or 48V DC to 240V AC/ 415V 3  ) - - Self synchronising (50Hz UK - 60Hz USA) - - Enables Islanded operation - - Costs from £1200 (approx) Issues - - Power Quality- Harmonics (transistor switching) - - DC injections from frequency synchronisation (heat dissipation at substation) - - Must isolate in the event of loss of network (safety) - - Potential for poor part load efficiency performance

Photovoltaics: Building integrated Photovoltaics: Building integrated Hybrid Photovoltaics (PV-Hybrid) Utilises both thermal and electrical power Treated as a flat plate collector with the following format: i) Power entering the system GA  ii) Electricity generated by the pv component GA  E iii) Power lost from component UA(T c -T a )

P max / GA Photovoltaics: Photovoltaics: Hybrid Photovoltaics iv)Thermal power supplied to system Q h = [GA  - GA  E ] - UA(T c -T a ) - = [GA (  -  E STC (1 - P p (T c - T ref )))] - UA(T c -T a ) Typical heat to power ratios of hybrid systems up to  4:1 Typical system efficiencies up to  75%

Photovoltaics: Photovoltaics: Hybrid Photovoltaics Passive cooling applications South Hybrid PV North

Research based: addressing control and power transfer issues via demonstration Photovoltaics: Photovoltaics: Solar cars