The High Efficiency Gas Boiler Engineering Essay

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Just before the UN climate talks started in Cancun on November the 29th 2010, the established research institute Maplecroft published a study (1) rating 183 countries on their CO2 emissions from energy use. The study indentified the Netherlands as the fifth worst performing nation in relation to CO2 pollution per capita. The Netherlands is the only European country that's rated 'extreme risk'.

Domestic hot water and heating accounts for nineteen percent of the Netherlands total CO2 emission. To attain the European mandate on CO2 reduction, the Dutch government introduced the EPC "Energieprestatiecoefficient" for newly built dwellings. The EPC is an index for the energetic efficiency of dwellings, taking into account the energy for heating, cooling, hot tap water and lightning in relation to the surface area of the dwelling. Since 2006 the EPC demand is 0,8, the following EPC demands have to be fulfilled in the near future:

2006, EPC = 0,8

2011, EPC = 0,6

2015, EPC = 0,4

2020, energy neutral

The high efficiency gas boiler has reached its maximum potential, therefore, innovative energy saving measures are required. Heat pumps are a technology which can contribute to low carbon heating and hot water provision in the domestic market.

In the next ten years, Ymere and the city Almere will develop the residential area Almere Hout Noord (figure 1). The target is building a CO2 neutral and energy independent neighborhood. The project is also marked as an excellent area and therefore has an EPC demand of 0,4. These ambitious targets require the input of several sustainable measures. However, the area has some limitations:

The building area's ground is used for making drinking water.

Because of the energy independent target, no natural gas network is installed

These limitations exclude the use of local ground source heat pumps and natural gas boilers. In order to fulfill the sustainable targets, an energy concept with air source heat pumps is investigated. This report aims to establish if air source heat pump concepts are worth to consider as a residential heating option and how to maximize its potential. It also describes the risks and do's and don'ts that have to be taken into consideration when applying air source heat pumps to dwellings.

Figure 1 - Location of Almere Hout Noord (marked yellow)

Section one describes the technology of air source heat pumps and its basic components. The operation of the heat pump is explained and the terms coefficient of performance (COP) and season performance (SPF) are introduced. A description of the CO2 emission reduction potential and energy bill saving potential is given.

Section two attends the other components of the energy concept that affect the performance of the heat pump and the energy efficiency of the dwelling. The "trias energetica" is introduced as a guide to reduce energy consumption. A description of the model house is given and the assumptions for isolation are outlined. This information is valuable in the context of the simulations that are made later in the study. Furthermore, the potential of the use of solar energy is considered and the possibility of using ventilation air as a heat source is described.

In section three the variables for comparing heat pumps with a simulation is explained and a basic simulation of heat pumps is made. The section starts with the weather files that are used and how variables like COP curves, heating curves and start/stops are used in the simulation. Then the optimal capacity dimension of the air source heat pump is determined by simulating different heat pump sizes. Based on the optimal heat pump capacity a comparison on energy use is made between heat pump brands and models.

Section four consists of the risks that come with using air source heat pumps for users, installers and developers. The risk of fluctuating energy bill is explained and how wrong usage or dimensioning leads to high energy use. Beside high energy use and energy bills, the risk of noise is explained and possibilities to control the risks are given. Also the impact on the electrical connection per dwelling and the area is considered.

In section five design directives are given for developers, architectures, advisors, building contractors and heat pump manufacturers. The section emphasizes the need for integrated design to guarantee the working of air source heat pump installations.

Finally section six will summarize the main findings from each section of the report and give recommendations for further research into the possibilities of air source heat pumps.

Ergens in de inleiding nog verwijzen naar de mindmap zoals Roelant adviseerde?

1 Air source heat pump technology

1.1 What is an air/water heat pump?

Although an air temperature of -10 °C is cold, it still contains heat. Everything above the absolute minimum temperature of - 273,15 °C contains heat that can be extracted. A heat pump is an electrical heating device that moves heat from a lower temperature heat source to a higher temperature heat distributor. This heat distributor can be a radiator, floor heating or hot water device. The air source heat pump uses outside air and/or ventilation air as heat source.

A refrigerant (or any liquid/gas) changes phase at different temperatures depending on the pressure. This principle makes it possible to evaporate at low temperature at a low pressure and liquefy at high temperature at a high pressure. Evaporating extracts energy from a lower temperature heat source and liquefying supplies it to a warmer heat distributor. Shown below in figure two is the basic representation of a heat pump cycle.

Figure 2 - Principle of a heat pump

The gas that enters the compressor (1) is compressed, which increases the pressure of the gas and the temperature of the gas. The hot gas then enters the condenser (2), which is basically a heat exchanger, where it releases its energy to the colder heat distributor. This makes the hot gas condense to a liquid. The liquid enters the expansion valve (3) where the pressure is decreased, and thereby decreasing the temperature and boiling point of the fluid. In the evaporator (4) the cold liquid absorbs energy from the warmer heat source, making it evaporate. Then the cycle starts again by compressing the gas with the compressor. The heating capacity is the energy extracted from the heat source plus the energy put into the refrigerant by the compressor.

1.2 COP and SPF

The compression cycle as described in figure 3 is usually represented on a pressure-enthalpy diagram (figure three below) since it is composed of two constant pressure process and one constant enthalpy process.

Figure 3 - Pressure enthalpy diagram with compression cycle

The main benefit of a heat pump can be shown through calculation.

Work in compression = h3 - h2 = 390 - 300 = 90 kJ/kg

Heating effect from condenser = h3 - h4 = 390 - 140 = 250 kJ/kg

COP = Useful Heat (Th)/ Work Expended (Th-Tc) = 250 / 90 = 2.77

Therefore for every one kW of energy used by the compressor 2.77kW of heat is generated. The efficiency of this process is called the coefficient of performance (COP). A more realistic COP however is the heat output divided by the total amount of electric energy consumed by the compressor and other electric components.

: Heat supplied

: Electric energy consumed

When calculating heat pump performance it is more important to consider its efficiency over time or Seasonal Performance Factor (SPF). Calculating this factor requires knowledge of the variable loads and source temperatures over time.

1.3 Types of operation modes

There are different types of heat generation operation used by air-water heat pumps.

Monovalent: The total amount of heat is generated by the heat pump.

Bivalent: The heat pump has an additional heater such as an electric heater or a high efficiency gas heater. The additional heater delivers heat when the capacity of the heat pump is insufficient or the efficiency of the gas heater is higher than the efficiency of the heat pump.

Because of the Almere Hout Noord restrictions, the only option is a monovalent heat pump or a bivalent heat pump using an additional electric heater (bivalent mono-energy).

Figure 4 - Bivalent mono-energy (left) and monovalent (right) operation

1.4 Reduction potential for CO2 and energy bill

Heat pumps cannot be considered a totally renewable energy technology. The compressor and some other components use electric energy which is usually generated with the use of fossil fuels. When the electricity is generated using renewable sources such as photo voltaic cells or wind turbines, the combination of technology is considered renewable.

The most used fossil fuel heating system for newly built dwellings in the Netherlands is a high efficiency natural gas boiler (HR107). A typical corner dwelling that is well insulated has an annual energy use for heating and hot tap water of circa 27 GJ. Using the CO2 emission factors given by SenterNovem (table 1) the amount of CO2 emitted is calculated.

Fuel

CO2-emission factor kg/MJ

Natural gas

0,0560

Electricity

0,0694

External heat delivery with coals

0,094

External heat delivery with oil

0,0730

Waste incineration

0,0314

Table 1 - Senter Novem emission factors used in EPC calculation using NEN 5128

For the natural gas boiler (annual efficiency of 85%) the amount of emitted carbon dioxide is:

For the air source heat pump operating for heating and domestic water (SPF is approximately 3 ) the amount of emitted carbon dioxide is:

The carbon dioxide reduction in this example is about 67% compared to a natural gas boiler. Another benefit is the cost saving on the annual energy bill.

Natural gas boiler annual energy cost.

Air source heat pump energy cost.

Annual cost savings on energy in this example is about 10%. The price for natural gas is expected to rise faster than the price for electricity (figure ), due to depletion of fossil fuels and the increasing use of renewable sources for electricity. This will make the potential for savings on energy cost increase. The required investment for a heat pump is, however, larger than a gas boiler. A complete comparison requires a total cost of ownership. This comparison is made in section three.

Figure 5 - Development of energy prices historical (solid lines) and forecast (marked)

2 Other components of the air source heat pump concept

2.1 Trias energetica as energy reduction guide

In 1996 SenterNovem introduced the Trias Energetica (figure 6) as a simple three-step plan to achieve the most sustainable energy management.

The three steps are:

Reduce unnecessary energy use, for example better insulation;

Use natural resources wherever possible at any level;

For the remaining energy need, use fossil fuels as efficient as possible;

Figure 6 - Trias Energetica

The houses built in Almere Hout Noord will be designed and built using this concept. Determining the optimal cost/reduction factor for insulation is not addressed in this report because of the project boundaries.

2.2 SenterNovem reference house

The final design and size of the dwellings build in Almere Hout Noord are unknown. This means the required energy for heating, which depends on size, isolation, windows, etc... can't be calculated. A commonly used method is making calculations based on the SenterNovem reference houses. Together with the standard Ymere values for better insulation and glass (Annex ??) this is the basis for heat transmission calculations.

The calculations are made with BINK software using the ISSO 51 calculation method. The totals heat loss at an outside temperature of -10 °C for the cornerhouse is little more than 9 kW and for the apartment almost 7 kW (Table 2).

Heat transmission at -10 °C

Room

SenterNovem corner house

SenterNovem apartment

Living room / kitchen

4 kW

3,2 kW

Bathroom

0,9 kW

1,2 kW

Entry

0

-

Hall

0

-

Bedroom 1

1,1 kW

1,3 kW

Bedroom 2

0,6 kW

0,9 kW

Bedroom 3

0,7 kW

-

Toilet

0,3 kW

0,2 kW

Loft

1,4 kW

-

Total

9,0 kW

6.8 kW

Table 2 - Heat loss Senter Novem reference houses

The heating curve (figure 7) shows the amount of heat that's needed at given temperature. This curve is used in section 3 for making heat demand simulations.

Figure 7 - (heating curve for the corner house and apartment)

2.2 Solar collector

The benefit of a solar collector depends on the size of the collector in relationship to the hot water demand. When the size of the collector increases, the amount of energy needed for hot water decreases. This relationship is expressed as the reduction factor. The growth of the reduction factor is smaller for every extra m² added. A larger contribution to the hot water demand requires more energy delivered at a darker period of the year (winter, autumn) when the efficiency of the collector is smaller. This reduces the total efficiency of the system per m².

Table 3 - Typical solar collector installation

For a corner house with four rooms the daily hot water demand is 100 liter (table ??). This is an annual hot water demand of 36,5 m³. The following formula from ISSO 59 calculates the needed collector area for a reduction factor.

Formule nog even invoegen van ISSO 59

20% = 20 x 36,5/1000 = 0,73 m²

30% = 40 x 36,5/1000 = 1,46 m²

40% = 60 x 36,5/1000 = 2,19 m²

50% = 95 x 36/1000 = 3,5 m²

The cost for the collector depends on the size of the collector, but the installing and extra components differ little. Because the system already has a buffer for hot water, no extra buffer is needed, which highly decreases the cost for solar collector installation.

Collector size

Installing

Pump and piping

Collector

Total

0,73 m²

€ 300,-

€ 300,-

€ 300,-

€ 900,-

1,46 m²

€ 300,-

€ 300,-

€ 500,-

€ 1.100,-

2,19 m²

€ 400,-

€ 300,-

€ 700,-

€ 1.400,-

3,5 m²

€ 500,-

€ 300,-

€ 1000,-

€ 1.800,-

Table 4 - Cost for solar collector installation

When a solar collector has a reduction factor of 50%, it means an average annual reduction of 50%. In the summer period the reduction could be 80% and in the winter period 20%. A heat pump uses less energy for the same amount of hot water at high outside air temperatures (summer) compared to low outside air (winter) temperatures. This means that an annual reduction of hot water demand from a heat pump doesn't direct translate to the same percentage reduced energy used by the heat pump.

Because the exact reduction factors for every month are not available, the effect of energy used in the worst case is calculated (table 4). When a reduction of 50% is wanted, the worst case for a heat pump would be 0% reduction during 6 cold months (low COP) and 100% reduction during 6 warm months (high COP).

Annual hot water reduction factor

Cold months reduction factor

Warm months reduction factor

Annual energy demand reduction factor

Difference

50%

0%

100%

45%

10%

50%

20%

80%

47%

6%

50%

50%

50%

50%

0%

40%

0%

80%

36%

10%

30%

0%

60%

27%

10%

20%

0%

40%

18%

10%

Table 5 - Actual reduction factor for air source heat pumps

The table shows that the maximum difference in reduction is 10% in worst case scenario for an air source heat pump. Taking into consideration a lifespan of 20 years and a discount rate of 5% the cost per reduced kWh is calculated.

Reduction

Collector size

Capital cost (20 years)

Reduced kWh (20 years)

€/kWh

18%

0,73 m²

€ 1.819,-

4.080 kWh

€ 0,45

27%

1,46 m²

€ 2.315,-

6.120 kWh

€ 0,38

36%

2,19 m²

€ 2.645,-

8.160 kWh

€ 0,32

45%

3,5 m²

€ 3.307,-

10.200 kWh

€ 0,32

Table 6 - Cost per reduced kWh with a solar collector

The cost to reduce a kWh used electricity by the heat pump is higher than the actual electricity price. This means the investment doesn't payback.

2.3 Photo voltaic

Moet ik nog uitzoeken

2.4 Heat delivery system

The conventional heating system for Dutch dwellings consists of a high efficiency natural gas boiler, a water pipe system, and radiators as heat supply. The radiators operate at a temperature of 90-70 °C. When using heat pumps in dwellings for heating, Low Temperature heating elements (40-30 °C) are needed. There are two types of LT heating elements, radiant heating and LT convectors. It's also possible to combine the two heating elements in one heating system.

2.4.1 Radiant heating

Radiant heating systems involve supplying heat directly to surface elements as floors, walls or ceilings. Although a high amount of heat is transferred by radiation, radiant heating systems also depends on convection. Because hot air has the natural tendency to raise, floor heating is more effective than wall or ceiling heating.

There are three types of radiant floor heaters: radiant air floors; electric radiant floors; hydronic (hot water) radiant floors. Most of the radiant systems are hydronic radiant floors (hot flowing fluid in pipes). The pipes are often cast into a concrete slab (figure ??), sometimes they are directly attached to the floor or wooden surface.

Figure 1 - Hydronic radiant floor

Figure 2 - Vertical temperature gradient

The two main advantages of underfloor heating are the increase in thermal comfort and the lack of space usage. Thermal comfort is affected by air temperature, mean radiant temperature, air velocity , relative humidity, conduction, clothing and metabolism. Underfloor heating affects mean radiant temperature (radiant asymmetry), air temperature (more steady) and conduction (feet on floor) positively.

One of the disadvantages of surface heating is the slow reaction to temperature changes. This is caused by the high amount of water in the piping and the thermal mass of the surrounding concrete. The slow reaction of radiant heating makes it unsuitable for bedrooms or the study, waiting a few hours before the room is at desired temperature is unacceptable.

The capacity of underfloor heating depends on the temperature and the distance between the piping. The maximum amount of heat at 35 °C is

(invoegen van de maximum afgifte van vloerverwarming. Schuif van Mark kopieren?)

Advantages

Thermal comfort;

Indoor air quality;

No space usage;

No service required;

Well known;

Costumers think of it as a luxury item ;

Disadvantages

High cost;

Slow reaction;

Energy waste due to slow reaction;

Limited amount of heat transferred

Unsuitable for bedrooms and study's

2.3.2 Low temperature heating convectors

Most of the heating power of heating convectors is by convection and just a little by radiation. Conventional convectors are designed for use with a 90/70 °C or 80/60 C° water circuit. This high temperature makes the air rise quickly through the convector and creates a natural air flow along the surface of the convector, increasing the heating power substantially.

When low heating temperatures are used (35 °C), the air rises slowly and thereby limiting the heating power greatly. Because of the low supply temperatures of air-water heat pumps new convectors are developed that are able to supply the needed heat at low temperatures. One of the leading manufacturers of such low temperature convectors is Jaga from Belgium. They developed the DBE series which uses (low energy) ventilators to boost the air through the convector.

Figure 8 - Reaction of LT convectors

Figure 9 - Reaction of radiant heating

Another advantage of the DBE convectors is the low water content. This means little time before the convector is at the desired temperature, and when the desired temperature is reached it stops heating almost immediately. and show the effect of difference in reaction time. The blue line is the set point and the red line is the measured temperature. Every minute the temperature is above the set point is wasted energy.

Advantages

Quick reaction;

Low energy waste;

Disadvantages

Space use;

Service required;

Noise;

New to most costumers;

2.3.3 Combination of floor heating with heating elements

Because of the maximum capacity of floor heating at 35 °C, the living room needs an additional heating element. A way to combine this two heating elements is giving the underfloor heating a set point of 18 °C. The LT convector tunes the last 2 degrees to 20 °C. This was there is the luxury of underfloor heating and the quick reaction of LT convector.

Figure 10 - Combination of floor heating with LT convectors

Because of the little time spend at bedrooms or study's, the LT convectors which react quickly to set points is advised. This increases the comfort and decreases the amount of energy needed for heating.

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