Exercises: Sustainable consumption and energy saving

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.

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.).

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 electricity bill does not charge for 'watts' but for '$\text{kW} \times \text{h}$'. 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, water heater, clothes dryer) and explain why electrical heating elements consume so much energy.

Many household appliances have an "eco" program. Explain what this program usually changes (lower temperature, longer duration, less water) and why it saves energy even though it takes longer to finish.

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.

Insulation is the primary measure for energy savings in heating and air conditioning. Explain what insulating a home means and name two areas through which the most heat escapes in winter (windows and shutter boxes, roof, exterior walls), giving a simple measure for each.

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 $\text{kW} \times \text{h}$ ($E = P \times t$). (b) Calculate the monthly savings in dollars if the price is $\$0.20$ per $\text{kW} \times \text{h}$. (c) Comment on why the savings from LED bulbs are so significant when there is barely any noticeable difference in lighting.

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 refrigerator consumes in a full year. (c) The refrigerator 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.

Propose a short "audit" of phantom energy consumption (standby power) in your home: how you would locate the appliances that consume the most on standby (observation, plug-in energy monitor, electric meter), which appliances you suspect would top the list, and what specific measures you would implement after measuring.

An electric water heater of $1500\ \text{W}$ operates for $3\ \text{h}$ a day. (a) Calculate its monthly consumption in $\text{kW} \times \text{h}$ (30 days). (b) Calculate its monthly cost at $\$0.20$ per $\text{kW} \times \text{h}$. (c) Calculate the monthly savings if usage is reduced to $2\ \text{h}$ per day and comment on how this reduction can be achieved (timer, lower setpoint temperature).

A clothes dryer of $2500\ \text{W}$ is used for $1\ \text{h}$ per load, 12 times a month. (a) Calculate its monthly consumption and cost at $0{,}\$20$ per $\text{kW} \times \text{h}$. (b) Compare this with hanging clothes out to dry (almost zero energy cost) and reason about which situations make using the clothes dryer justified (damp winter, lack of space, allergies) even if it costs money.

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

It is estimated that every degree lower in the heating setpoint saves around $7\ \%$ in energy. If a family spends $\$100$ a month on heating, calculate the approximate monthly savings of lowering the thermostat from $22\ ^\circ\text{C}$ to $20\ ^\circ\text{C}$ and comment on what comfort measures offset those two degrees (warm clothing, blankets, draft stoppers).

"Leaving the cell 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 $0{,}\$20$ per $\text{kW} \times \text{h}$ (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.

A user claims: “My 10-year-old refrigerator still works perfectly, but it consumes $380\ \text{kW} \times \text{h}/\text{año}$. I am going to throw it away today to buy a class A one ($130\ \text{kW} \times \text{h}/\text{año}$, $\$600$) because that way I will save money and immediately reduce my ecological footprint”. You have the following technical data: the cost of electricity is $0{,}\$22/(\text{kW} \times \text{h})$, 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 $1\,100\ \text{kW} \times \text{h}$ 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 planned obsolescence.

A family lives in a $90\ \text{m}^2$ apartment in a cold climate zone. Their annual consumption is $8\,500\ \text{kW} \times \text{h}$ for Joule-effect electric heating, $2\,400\ \text{kW} \times \text{h}$ for domestic hot water (electric water heater), and $2\,800\ \text{kW} \times \text{h}$ for appliances and lighting, paying for electricity at $0{,}\$20/(\text{kW} \times \text{h})$. They have a fixed budget of $\$3\,000$ 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 ENERGY STAR-certified units (investment: $\$2\,200$; estimated savings: $25\ \%$ of that category).

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

3. Replace the electric water heater with a heat pump water heater for domestic hot water (investment: $\$2\,400$; seasonal average efficiency $\text{COP} = 3.0$).

(a) Calculate the annual energy savings (in $\text{kW} \times \text{h}$) and financial savings (in dollars) 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 a priority compared to replacing active generation systems.

A city government subsidizes 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 $300\ \text{kW} \times \text{h}$ to $225\ \text{kW} \times \text{h}$, well below the savings predicted by the city 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.

An energy audit in a detached house over one month records the following electricity consumptions: heat pump heating $210\ \text{kW} \times \text{h}$, water heater $135\ \text{kW} \times \text{h}$, refrigerator $45\ \text{kW} \times \text{h}$, electric stovetop $40\ \text{kW} \times \text{h}$, standby power consumption $20\ \text{kW} \times \text{h}$, and lighting $15\ \text{kW} \times \text{h}$. The family proposes focusing all their efforts on completely eliminating standby consumption and replacing all light bulbs with ultra-low-energy models, claiming that "this will drastically cut their bill".

(a) Critically analyze the family's claim: calculate the maximum percentage of the total monthly consumption represented jointly by standby power and lighting, and demonstrate why this approach suffers from the scale fallacy when compared to a Pareto analysis (80/20 rule).

(b) The $100\ \text{l}$ electric water heater keeps the water at $65\ ^\circ\text{C}$ continuously, recording static thermal losses through insulation of $1.2\ \text{kW} \times \text{h}/\text{day}$. Design two technical interventions on the water heater (one involving timer/thermostatic regulation and another on flow consumption) and quantify the percentage savings and savings in $\text{kW} \times \text{h}$ that would be achieved in this category.

(c) Technically justify why a $15\ \%$ reduction in the two main thermal categories generates a much greater economic and environmental impact than eliminating $100\ \%$ of the minor categories.

A small technical office replaces its old climate control and lighting systems with top-rated energy efficiency equipment. However, during the first few months, the electricity bill barely decreased: users kept the thermostat at $19\ ^\circ\text{C}$ in summer and at $24\ ^\circ\text{C}$ in winter, and fixtures remained switched on for hours in unoccupied offices and meeting rooms under the premise that "since they are low-consumption LEDs, the expense is negligible".

(a) Mathematically model the thermal impact: knowing that for each degree the thermostat setpoint deviates from the standard comfort temperature ($26\ ^\circ\text{C}$ in summer and $21\ ^\circ\text{C}$ in winter, according to ASHRAE), the heat pump's consumption varies by approximately $7\ \%$, calculate the percentage extra cost resulting solely from improper setpoint habits in summer and winter.

(b) Design a technical solution based on automation and control (PIR occupancy/light sensors, smart thermostats locked by software with deadband, and centralized scheduling) to correct the problem without relying on individual user willingness.

(c) Technically justify why in energy engineering passive automation (systems that operate autonomously) is prioritized over awareness campaigns to neutralize the rebound effect.