Sustainable consumption and saving energy

The energy label on household appliances rates them with letters from A to G. Explain what information this label provides and why a class A appliance can end up being cheaper than a class G appliance even if it costs more to buy.

The energy label of one washing machine indicates $220\ \text{kW} \cdot \text{h}$ per year and another indicates $160\ \text{kW} \cdot \text{h}$. (a) Calculate the annual cost of each at $0{,}20\ \text{€}$ per $\text{kW} \cdot \text{h}$. (b) Calculate the savings over a 10-year lifespan. (c) Reason how you would use these figures alongside the purchase price to choose between the two.

An electricity bill does not charge for 'watts' but for '$\text{kW} \cdot \text{h}$'. Explain with an example the difference between the power rating of an appliance and the energy it consumes (a $800\ \text{W}$ microwave used for 15 minutes versus a $10\ \text{W}$ light turned on all day).

Appliances that produce heat are the biggest energy consumers. Name three typical 'power guzzlers' in a home (oven, electric heating, hot water heater, tumble dryer) and explain why electrical heating elements consume so much energy.

The figure compares the monthly electricity consumption of several household appliances, as shown in the figure.

(a) Read the monthly consumption of each appliance from the graph. (b) Calculate how much energy the fridge consumes in a full year. (c) The fridge consumes a lot of energy even though its power rating is modest, whereas the oven is very powerful but consumes less per month: explain this apparent contradiction and reason which two appliances energy-saving efforts should be focused on.

Explain what standby power consumption is in appliances, why it is sometimes called 'phantom load', and describe two practical ways to eliminate it at home without giving up using the appliances.

An appliance on standby consumes between $1\ \text{W}$ and $5\ \text{W}$ without doing anything useful. Explain how it is possible for it to consume energy while "turned off" (plugged-in power supply, indicator lights, waiting for the remote control signal) and how this power consumption can be eliminated completely.

"Leaving the mobile phone charger plugged in uses a huge amount of energy." Put this into context with numbers: an idle charger consumes between $0{,}1\ \text{W}$ and $0{,}5\ \text{W}$. Calculate the annual cost in the worst-case scenario at $0{,}20\ \text{€}$ per $\text{kW} \cdot \text{h}$ (a year has $8760\ \text{h}$) and conclude where energy-saving efforts should be directed (heating appliances, standby of many appliances simultaneously) as opposed to actions with minimal impact.

An incandescent light bulb of $60\ \text{W}$ is replaced with an LED bulb of $9\ \text{W}$ that provides the same amount of light, used for $5\ \text{h}$ a day for $30$ days. (a) Calculate the monthly energy consumed by each, in $\text{kW} \cdot \text{h}$ ($E = P \cdot t$). (b) Calculate the monthly savings in euros if the price is $0{,}20\ \text{€}$ per $\text{kW} \cdot \text{h}$. (c) Comment on why the savings from LED bulbs are so significant when there is barely any noticeable difference in lighting.

A user claims: “My 10-year-old refrigerator still works perfectly, but it consumes $380\ \text{kW} \cdot \text{h}/\text{año}$. I am going to throw it away today to buy a class A one ($130\ \text{kW} \cdot \text{h}/\text{año}$, $600\ \text{€}$) because that way I will save money and immediately reduce my ecological footprint”. You have the following technical data: the cost of electricity is $0{,}22\ \text{€}/(\text{kW} \cdot \text{h})$, the estimated lifespan of the new refrigerator is 10 years, and manufacturing, transporting, and recycling a new refrigerator involves an embodied energy footprint equivalent to $1\,100\ \text{kW} \cdot \text{h}$ of primary energy. (a) Quantitatively demonstrate whether the user's economic claim is correct or incorrect by calculating the payback period of the investment and the net financial balance at the end of the lifespan. (b) Determine from which year the overall energy balance (manufacturing plus accumulated operation) begins to favor replacement. (c) As a future technologist, write a reasoned recommendation: is it advisable to replace it right now or wait until it breaks down? Justify the concept of ecological payback versus voluntary obsolescence.

A school intends to replace 120 fluorescent light fixtures with $2 \times 36\ \text{W}$ tubes (conventional electromagnetic ballast) with $40\ \text{W}$ LED panels. The caretaker states: "The power savings will be exactly $(72 - 40)\ \text{W} = 32\ \text{W}$ per fixture, and since the LEDs have $\cos \varphi = 0{,}95$ compared to $\cos \varphi = 0{,}50$ for the uncompensated tubes, we will also halve the active energy billed on the meter." The lights operate $1\,200\ \text{h}$ per year, each ballast dissipates an additional $9\ \text{W}$ of constant thermal losses, and the cost of active $\text{kW} \cdot \text{h}$ is $0{,}18\ \text{€}$.

(a) Refute the caretaker's claim about the meter and the power factor using technical arguments: what does the active energy meter measure, and what electrical quantity is actually reduced by improving $\cos \varphi$?

(b) Calculate the real active power demanded by each fluorescent fixture (tubes plus ballast) and determine the school's total real annual energy savings (in $\text{kW} \cdot \text{h}$) and financial savings after switching to LEDs.

(c) If each LED panel costs $25\ \text{€}$ installed, calculate the simple payback period. Predict what will happen to the building's thermal demand in winter and in summer due to the reduction in waste heat dissipated by the fixtures (thermal interaction effect).

Propose three simple actions to save energy in a home (lighting, heating/air conditioning, appliance use, etc.) and explain for each one why it saves energy and which quantity is reduced (power, operating time, losses, etc.).

A family lives in a $90\ \text{m}^2$ flat in a cold climate zone. Their annual consumption is $8\,500\ \text{kW} \cdot \text{h}$ for Joule-effect electric heating, $2\,400\ \text{kW} \cdot \text{h}$ for domestic hot water (electric water heater), and $2\,800\ \text{kW} \cdot \text{h}$ for appliances and lighting, paying for electricity at $0{,}20\ \text{€}/(\text{kW} \cdot \text{h})$. They have a fixed budget of $3\,000\ \text{€}$ for immediate renovations and must choose between three options that are mutually exclusive due to the budget:

1. Replace all lighting with LEDs and appliances with class A units (investment: $2\,200\ \text{€}$; estimated savings: $25\ \%$ of that category).

2. Insulate the false ceiling and roller shutter boxes, and install weatherstripping and low-emissivity films on windows (investment: $1\,800\ \text{€}$; estimated reduction of $20\ \%$ in heating thermal demand).

3. Replace the electric water heater with an aerothermal heat pump for DHW (investment: $2\,400\ \text{€}$; seasonal average efficiency $\text{COP} = 3{,}0$).

(a) Calculate the annual energy savings (in $\text{kW} \cdot \text{h}$) and financial savings (in euros) for each of the three options.

(b) Determine the payback period of the investment for each option and technically justify which one should be implemented first with the available budget.

(c) Explain why taking action on passive demand (thermal insulation) is usually considered technically prioritary compared to replacing active generation systems.

A town hall subsidises the replacement of old air conditioning units (seasonal efficiency $\text{EER} = 2{,}0$, average electrical input power $1{,}5\ \text{kW}$) with new high-efficiency inverter units ($\text{EER} = 4{,}0$). It was estimated that each family used the old unit for $200\ \text{h}$ per summer. Following the campaign, a field study reveals that the average summer electricity consumption per household only decreased from $300\ \text{kW} \cdot \text{h}$ to $225\ \text{kW} \cdot \text{h}$, well below the savings predicted by the municipal technicians.

(a) Calculate the electrical consumption that the technicians had predicted for the new units, assuming exactly the same cooling thermal energy delivered and the same $200\ \text{h}$ of operation.

(b) Quantitatively demonstrate the existence of the rebound effect by calculating how many actual operating hours the new units were run for and what percentage of the theoretical potential savings has been lost due to the change in habits ("rebound rate").

(c) Justify what socio-technological or user perception factor causes this deviation, and propose two measures (one technical measure based on automation/smart home systems and one economic measure) to prevent the efficiency gain from being diluted.