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Qwestrum Engineering360 · Mechanical Engineering · Renewable Energy

Solar Thermal and PV

Solar PV output is P = η_module·G·A (G = irradiance, A = area); solar thermal collectors capture heat with efficiency depending on losses. PV converts light directly to electricity, thermal converts it to heat, per renewable-energy texts.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • Flat plate vs evacuated tube collectors for thermal
  • MPPT maximises power from varying irradiance
  • Temperature coefficient reduces η at high cell temperature

Topic details

Introduction

Solar energy is captured either as heat (solar thermal) or directly as electricity (photovoltaics), both important in India's renewable push. Renewable-energy courses cover collector performance and PV cell characteristics.

Scope in B.Tech and GATE syllabus

Solar thermal collectors (flat-plate, evacuated-tube, concentrating) absorb irradiance and transfer heat to a fluid; efficiency falls as operating temperature rises due to greater losses. Applications range from water heating to concentrated solar power.

Why this topic matters in practice

PV modules convert a fraction (module efficiency) of incident irradiance to DC electricity; output scales with irradiance and area and falls with temperature. Computing PV output and collector efficiency, and understanding the I-V curve, are the exam skills.

Key relations & formulas

PPV=ηmodule⋅G⋅AP_{PV} = \eta_{module} \cdot G \cdot A
(G = irradiance W/m², A = panel area)
ηmodule≈15−22\eta_{module} \approx 15-22%
(commercial silicon)

Formulas (Indian textbook notation)

  • Q_{solar}_collector = ṁ\cdot c_{p}\cdot (T_{out} - T_{in}) = \tau\alpha\cdot G\cdot A - U_{L}\cdot A\cdot(T_{m} - T_{a})

Formulas (Indian textbook notation)

  • CapacityfactorCF=actualoutputratedoutputoverperiodCapacity factor CF = \frac{actual_{output}}{rated_{output}} over period

Notation and sign conventions

Relation 1 —
PPV=ηmodule⋅G⋅AP_{PV} = \eta_{module} \cdot G \cdot A
PPV=ηmodule⋅G⋅AP_{PV} = \eta_{module} \cdot G \cdot A
(G = irradiance W/m², A = panel area)
Write this relation with symbols exactly as in Non-Conventional Energy Sources — GD Rai before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
ηmodule≈15−22\eta_{module} \approx 15-22%
ηmodule≈15−22\eta_{module} \approx 15-22%
(commercial silicon)
Write this relation with symbols exactly as in Non-Conventional Energy Sources — GD Rai before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
Q_{solar}_collector = ṁ\cdot c_{p}\cdot

Formulas (Indian textbook notation)

  • Q_{solar}_collector = ṁ\cdot c_{p}\cdot (T_{out} - T_{in}) = \tau\alpha\cdot G\cdot A - U_{L}\cdot A\cdot(T_{m} - T_{a})
Write this relation with symbols exactly as in Non-Conventional Energy Sources — GD Rai before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 4 —
CapacityfactorCF=actualoutputratedoutputoverperiodCapacity factor CF = \frac{actual_{output}}{rated_{output}} over period

Formulas (Indian textbook notation)

  • CapacityfactorCF=actualoutputratedoutputoverperiodCapacity factor CF = \frac{actual_{output}}{rated_{output}} over period
Write this relation with symbols exactly as in Non-Conventional Energy Sources — GD Rai before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Fundamentals and definitions

Photovoltaic cells generate electricity via the photovoltaic effect: photons free charge carriers across a p-n junction. Module output P = η_module·G·A, where G is plane-of-array irradiance (W/m²) and A the area; η typically 15–22 %.

Governing relations in practice

The PV I-V curve has a short-circuit current, open-circuit voltage, and a maximum-power point (MPP) at their optimal product; fill factor measures curve "squareness". Output rises with irradiance but falls as cell temperature increases (voltage drops), so real output is below nameplate on hot days.

Design and analysis considerations

Solar thermal collectors absorb irradiance and deliver useful heat Q = A[G·(τα) − U_L(T_c − T_a)]; efficiency η = Q/(G·A) drops as collector temperature T_c rises above ambient because loss U_L(T_c − T_a) grows. Concentration raises the achievable temperature for power generation.

Advanced theory and extensions

PV suits distributed electricity; solar thermal suits heat and, via concentration, large-scale power with storage. Matching technology to the energy need (electricity vs heat) and computing output are the applied skills.

Assumptions and validity limits

State assumptions explicitly before using any relation for solar thermal and pv — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Renewable Energy viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Renewable Energy papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to solar thermal and pv.
4. Use equation 1:
PPV=ηmodule⋅G⋅AP_{PV} = \eta_{module} \cdot G \cdot A
.
5. Use equation 2:
ηmodule≈15−22\eta_{module} \approx 15-22%
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Solar Thermal and PV appears in grid-connected and off-grid projects. In Indian mechanical curricula this topic is tested because it connects theory to solar, wind, and biomass energy systems.
GATE and semester exams often combine solar thermal and pv with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use solar thermal and pv?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Using nameplate efficiency without derating for temperature and losses
• Confusing irradiance G (W/m²) with insolation (energy, Wh/m²)
• Ignoring the maximum-power-point when estimating PV output
• Assuming thermal-collector efficiency is constant with operating temperature

Quick revision checklist

Before attempting solar thermal and pv problems, confirm you can:
1. Flat plate vs evacuated tube collectors for thermal
2. MPPT maximises power from varying irradiance
3. Temperature coefficient reduces η at high cell temperature
Revise the solved examples in Non-Conventional Energy Sources — GD Rai and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

PV module output

Problem

A PV array of area A = 20 m² has module efficiency 18 % under irradiance G = 800 W/m². Find the electrical power output.

Solution

P = η·G·A = 0.18 × 800 × 20 = 2880 W ≈ 2.88 kW.

Conceptual check — Solar Thermal and PV

Problem

In a Renewable Energy semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of solar thermal and pv." What should a complete answer include?

Practice questions

Most-asked interview and GATE questions for this topic — expand any item for a model answer.

  1. 1
    What is Solar Thermal and PV, and why does it appear in B.Tech / GATE syllabi?

    Model answer

    Solar PV output is P = η_module·G·A (G = irradiance, A = area); solar thermal collectors capture heat with efficiency depending on losses. PV converts light directly to electricity, thermal converts it to heat, per renewable-energy texts.
  2. 2
    State the relation P_PV = η_module · G · A and name each symbol.

    Model answer

    The governing relation is PPV=ηmodule⋅G⋅AP_{PV} = \eta_{module} \cdot G \cdot A. Write every symbol with SI units before substituting numbers.
  3. 3
    State the relation η_module ≈ 15–22% and name each symbol.

    Model answer

    The governing relation is ηmodule≈15−22\eta_{module} \approx 15-22%. Write every symbol with SI units before substituting numbers.
  4. 4
    State the relation Q_solar_collector = ṁ·c_p· and name each symbol.

    Model answer

    The governing relation is Q_{solar}_collector = ṁ\cdot c_{p}\cdot. Write every symbol with SI units before substituting numbers.
  5. 5
    State the relation Capacity factor CF = actual_output/rated_output over period and name each symbol.

    Model answer

    The governing relation is CapacityfactorCF=actualoutputratedoutputoverperiodCapacity factor CF = \frac{actual_{output}}{rated_{output}} over period. Write every symbol with SI units before substituting numbers.
  6. 6
    Explain: Flat plate vs evacuated tube collectors for thermal

    Model answer

    Flat plate vs evacuated tube collectors for thermal — state the assumption range and one exam trap linked to this point.
  7. 7
    Explain: MPPT maximises power from varying irradiance

    Model answer

    MPPT maximises power from varying irradiance — state the assumption range and one exam trap linked to this point.
  8. 8
    Explain: Temperature coefficient reduces η at high cell temperature

    Model answer

    Temperature coefficient reduces η at high cell temperature — state the assumption range and one exam trap linked to this point.
  9. 9
    How would you correct this error in a viva: Using nameplate efficiency without derating for temperature and losses?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  10. 10
    How would you correct this error in a viva: Confusing irradiance G (W/m²) with insolation (energy, Wh/m²)?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  11. 11
    How would you correct this error in a viva: Ignoring the maximum-power-point when estimating PV output?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  12. 12
    How would you correct this error in a viva: Assuming thermal-collector efficiency is constant with operating temperature?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.

Exams & GATE

  • 1
    GD Rai Ch. 1–4 — shading and tilt angle affect annual energy yield.
  • 2
    Avoid: Using nameplate efficiency without derating for temperature and losses
  • 3
    Avoid: Confusing irradiance G (W/m²) with insolation (energy, Wh/m²)
  • 4
    Avoid: Ignoring the maximum-power-point when estimating PV output

📖 Standard books (India)

  • Non-Conventional Energy Sources — GD Rai

    Read: Syllabus unit

    Solar, wind, and biomass — standard Indian text

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