Energy-Efficient Solutions for Office Buildings: Calculations and Physics

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Knowledge Base · Energy Efficiency

Engineering calculation of heat loss, HVAC efficiency, and lighting. Real-world implementation cases. References to EN 10077, ASHRAE 90.1, CIBSE Guide A.

U-value

0.35–0.8

W/(m²·K)

COP / EER

2.8–4.2 / 3.2+

heat pumps

Recovery

η ≥ 75%

ASHRAE 90.1

LED

≤ 8 W/m²

EN 12464-1

01 · Material Physics      

Heat Loss Through Building Envelope

The thermal transmittance coefficient U (W/(m²·K)) determines how much heat passes through 1 m² of surface at a temperature difference of 1 K. For typical materials:

Mineral wool: λ = 0.035–0.042 W/(m·K)
Expanded polystyrene: λ = 0.031–0.038 W/(m·K)
Triple-glazed unit with argon: U = 0.6–0.8 W/(m²·K) (EN 10077-1:2018)

Q = U × A × ΔT × t

where Q — heat loss (kWh), A — area (m²), ΔT — temperature difference (°C), t — time (h).

Example. For a wall of 50 m² with U = 0.35 W/(m²·K), ΔT = 25 °C over one day: Q = 0.35 × 50 × 25 × 24 / 1000 = 10.5 kWh/day.

02 · Thermal Physics of HVAC      

Heating, Ventilation, and Air Conditioning

Key indicators are COP (Coefficient of Performance) for heat pumps and EER for air conditioners.

COP = Q_heat / W_electric
 EER = Q_cooling (BTU/h) / P_input (W)

According to Eurovent test data, for Central European climates, air-source heat pumps have COP in the range of 2.8–4.2. ASHRAE 90.1-2022 recommends for offices a minimum heat recovery efficiency of 65–75%.

⚡ Paradox: Improving wall insulation without upgrading ventilation can increase the share of ventilation losses to 50–60% of the total balance. Investments in insulation become inefficient without simultaneous implementation of heat recovery.

03 · Engineering Calculations      

Measure Efficiency

3.1. Window Replacement and Insulation

Initial data: office space 100 m², glazing area 30 m², current U_window = 2.0, planned U_window = 1.0 (EN 10077). With ΔT = 25 °C, t = 24 h, daily savings:

ΔQ = (2.0 – 1.0) × 30 × 25 × 24 / 1000 = 18 kWh/day

For the heating season (180 days) — 3240 kWh. At a tariff of €0.12/kWh, savings amount to €388.8/year.

3.2. Heat Recovery from Ventilation Air

The cost of heating supply air is calculated by:

Q_vent = V × ρ × c × ΔT × t

For an office with V = 1000 m³/h, ΔT = 25 °C, t = 10 h/day, without recovery heat loss ≈ 83.75 kWh/day. With a recovery efficiency of 75%, residual losses are 20.9 kWh/day. Savings — 62.85 kWh/day, over the season — 11,313 kWh, or €1,357.6/year.

3.3. Lighting: Switch to LED

E_annual = (P_lamp × N × t × K_safety × 250) / 1000

Example: 100 fluorescent luminaires of 40 W replaced by LED of 18 W, t = 10 h/day. Savings:

(40 – 18) × 100 × 10 × 250 / 1000 = 5,500 kWh/year (€660/year)

04 · Field Evidence      

Implementation Cases

🇬🇧 London, UK

Scale: 20 storeys, 40,000 m², glazing 55%.

Measures: windows U=0.7; recovery 78%; full LED.

Result: 110 → 73 kWh/(m²·year).

🇩🇰 Copenhagen, Denmark

Scale: 8 floors, 15,000 m², district heating.

Measures: insulation U=0.22; rotary exchangers 72%; LED 85%.

Result: heat -45%, electricity -34%.

05 · Technical Clarifications      

Frequently Asked Questions

1. How does thermal conductivity affect the overall thermal resistance of a wall?

R_total = R_si + Σ(d_i/λ_i) + R_se, where λ_i is thermal conductivity. The smaller λ, the higher R. U = 1/R_total.

2. Why does COP vary during the year and how to account for it?

COP depends on source temperature. Use seasonal COP (SCOP) per EN 14825; for temperate climates it is 3.2–3.8.

3. How to estimate ventilation heat loss without an airflow meter?

Use air change rate per ASHRAE 62.1: for offices 1–1.5 ACH. Uncertainty up to 20%.

📚 Sources & References

rfid.org.ua · Knowledge Base · 2026-03-06   Standards: EN 10077 · ASHRAE 90.1 · CIBSE Guide A

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