Space heating is the most important energy use in residential buildings in cool and temperate climate zones. Energy-efficient heating systems can save up to 30% of primary energy and, if using renewable energy or waste heat, up to 85% of greenhouse gas (GHG) emissions. The strategic approach for selecting an appropriate heating system should be: (1) reduce the building’s heating demand through passive design options, (2) choose an energy-efficient heating system with low GHG emissions and if possible with cogeneration of heat and power, (3) - if possible - supplied or supported by local renewable energy resources (solar energy, geothermal or ambient heat, sustainable biomass).
Both in cool and in temperate climate zones, space heating is usually responsible for the biggest share of the final energy consumption and related greenhouse gas emissions in the residential building sector (see figure below).
There is wide variety of heating systems to meet the heating demand (see figure below for an overview), all with different energy efficiency, fuels, costs, and greenhouse gas emissions. Generally, cogeneration of heat and power is most energy-efficient option. Systems based on renewable energies should be preferred when aiming to construct zero or plus energy buildings. Many technological options can fundamentally be specified for different fuels. For example, low temperature or condensing boilers are available for the operation with natural gas, fuel oil, plant oil, biogas, or wood pellets respectively. Steam turbines can be powered by heat from combustion processes (e.g. combustion of gas, oil or biomass) as well as from concentrating solar radiation or geothermal heat. Note that some of the listed technologies - like geothermal power plants - are only feasible on a medium or large scale. Thus the choice for such options is not in the responsibility of a single building owner but rather a political and economic decision on municipal level. Furthermore it has to be stressed that the choice of the fuel or heating source of course depends on the local availability of fossil and renewable energy resources. In terms of plant size, infrastructural aspects and involved stakeholder it is useful to differ between three different groups of heat supply options.
Individual supply refers to the supply of a detached single-family house or an accommodation unit within a semi-detached house, or of a terraced house, or of an apartment building.
Block supply refers to the supply of one or more complexes of buildings, e.g. a semi-detached house, a multiple dwelling unit or a row of terraced houses, by a single central feeding unit. If a whole residential area is supplied by one medium scale heating system the term Local heating is also used in this context.
District heating refers to the supply of whole quarters or cities by large-scale heat supply stations. In comparison to local heating, in this case far distances have to be covered with heat transport pipes.
Within each group several technical heat supply options can be found. This section presents a selection of relevant and future-orientated technological options. They differ particularly in terms of energy efficiency, specific costs, heat to power ratio (for CHP plants) and specific CO2 emissions and are dealt in detail in the corresponding sub categories.
Energy efficiency of heat generation systems play a major role both in terms of ecology and in economy. A suitable criterion to assess the overall energy efficiency is the total primary energy demand. Within a holistic approach not only the energy efficiency of the heat generation system itself but also the supply chain for the used fuels is factored in. The primary energy demand can be distinguished between a renewable and a non-renewable (or fossil) share. Low primary energy demands can be achieved by highly energy efficient technologies (like condensing boiler), by the use of waste heat (CHP systems) or heat recovery (ventilation system) or by the use of renewable energies like solar radiation or ambient or geothermal heat. The following figure gives an overview over the primary energy demand of twelve uncoupled heating systems and three combined heat and power (CHP) systems.
The calculations of the primary energy demand largely depend on the local conditions for the electricity generation and for the fuel production, transformation and transport. The used primary energy coefficients for the calculations in the figure above are documented in the table below.
| Types of energy | Primary energy coefficients | Primary energy coefficients | Efficiency factor (reciprocal value) | Efficiency factor (reciprocal value) |
|---|---|---|---|---|
| - | Total | Non renewable share | Total | Non renewable share |
| Oil / gas | 1.1 | 1.1 | 0.91 | 0,91 |
| Wood | 1.2 | 0.2 | 0.83 | 5.00 |
| Fossil fuel (Local / district heating from CHP ) | 0.7 | 0.7 | 1.43 | 1.43 |
| Renewable fuel (Local / district heating from CHP ) | 0.7 | 0.0 | 1.43 | - |
| Electricity (German power plant mix) | 3.0 | 2.6 | 0.33 | 0.38 |
| Environmental energy (Solar energy, Environmental heat) | 1.0 | 0.0 | 1.00 | - |
As it can be seen electric resistance heating is the most inefficient and the less environmentally friendly kind of heating. Although the efficiency of the heat generator itself is 100 %, due to the losses in the upstream chain a primary energy demand of 263 % is needed for the supply of one kilowatt-hour of useful heat energy. By contrast electric heat pumps are considerable more effective as they additionally feed in 1.6 to 2.9 units of renewable energy per unit of used electric energy. Depending on the temperature of the used heat source (here: geothermal or ambient heat) and depending on the necessary flow temperature of the heating system electric heat pumps achieve seasonal performance factors (SPF) of about 2.6 (air-water-system in an existing building) to 3.9 (brine-water-system in a new building). Consequently they have a primary energy demand in a range of about 65 to 100 % per kilowatt-hour useful heat energy. This is considerable better compared to the energy demand of today’s conventional best available technology of a condensing boiler (110 %) or compared to the less efficient low temperature boiler (140 %). Wood pellets or wood chip heating systems (140 %) and especially split log ovens (180 %) have also a high total energy demand, but with 23 % to 30 % a very low non-renewable share. Thus wood heating systems are also environmentally sound, but only if the used wood is sustainably cultivated. Gas heat pumps are an alternative to electric heat pumps. They have lower seasonal performance factors, but incur fewer losses in the upstream chain. In total they need a similar amount of primary energy compared to the electric version. Gas heat pumps have been distinguished into as driven by a gas motor (SPF: 1.6 / PE demand: 69 %) and driven by heat through absorption or adsorption processes (SPF: 1.35 / PE demand: 81 %). In low energy buildings (LEB) or ultra-low energy buildings (ULEB) the heat losses caused by transmission through the building’s envelope are reduced so far, that the residual ventilation losses are in the same dimension. In this case mechanical ventilation systems with heat recovery (HRV) can be a smart solution. Those systems can recover 80 % and more of the ventilation losses. High efficient electronically commutated ventilation motors allow with one Kilowatt-hour of electricity the recovery of 10 Kilowatt-hours and more of heat energy (εel ≥ 10). If we assume a relation of 50/50 for a building’s transmission / ventilation losses and a heat recovery factor of 80% the HRV system can deliver 40 % of the total useful heat demand. The residual heating demand of 60 % must be provided by a backup-system like a small condensing boiler or a small air heat pump. The so called compact heat pump unit combines a HRV system with an exhaust-air heat pump in one device and needs in total 74 % primary energy per Kilowatt-hour of useful heat. The combination of a HRV system with a condensing boiler is with 76 % PE demand of almost the same efficiency.
The combined production of heat and electricity in one process, also known as combined heat and power (CHP), is a very energy efficient way of supplying energy. Depending on technology and size different thermal and electric efficiency factors and power-to-heat ratios can be achieved. Three examples in the figure above show CHP systems with low (stirling engine: 0.12), medial (internal combustion engine: 0.61) and high (fuel cell: 1.72) power-to-heat ratios. They produce 12 % (stirling), 61 % (ICE) and 172 % (fuel cell) electricity per Kilowatt-hour useful heat energy and obtain a primary energy credit of 37 %, 184 % and 522 % for the substitution of electric energy from the grid. Taking the natural gas input into account the stirling engine system has a total PE demand of 86 %, the ICE system of 12 % and the fuel cell system of -144 %. Keep in mind that these calculations have been made with an average (non renewable) electric efficiency factor of 0.33 for the substituted electricity from the grid. The considered CHP systems would have the same PE demand of 110 % like the reference system of a condensing boiler if the assumed efficiency of the electricity generation mix would improve from 33 % to 92 % (for stirling), to 70 % (for ICE) or to 64 % (for fuel cell). For comparison, today’s best available fossil electricity generation system is a natural gas combined cycle plant with an electric output of more than 500 MW and an electric efficiency of about 60 %. A completely renewable generation mix would be calculated with an efficiency factor of 100 %.
The following figure compares the greenhouse gas (GHG) emissions of different modern heating systems in relation to the provision of one kilowatt-hour of useful heat energy. Accordingly, the greenhouse gas emissions may already be reduced through natural gas condensing boilers compared to natural gas low temperature boilers by 15% and compared to fuel oil low temperature boilers by 34%. These savings alone are not sufficient to counter future resource scarcity and to achieve the ambitious climate protection goals. It is therefore necessary to develop and apply innovative, more energy efficient and low greenhouse gas emission heating technologies.
As shown in the figure the largest GHG emission reductions can be achieved by fully renewable supply options such as biogas in condensing boilers (- 61% vs. reference technology) and wood pellet heating (- 85%) . Solar collectors for supporting water and space heating can cover in a conventional new residential building between 20% and 30%, and in an ultra-low energy building up to 70% of the heat demand. In principle, they can be reasonably combined with any heating system other than CHP. In combination with a natural gas condensing boiler, a solar system in a building with low-energy building standard can typically deliver one third (37%) of the heat for heating and hot water supply. This makes possible a reduction of GHG emissions by around 30%. Electric or gas fired heat pumps provide another opportunity to integrate renewable heat. When judiciously applied 20 to 30% (based on the required primary energy use and depending on the power mix) of renewable energy in the form of geothermal or environmental heat can be used with these systems. The GHG savings will amount to approximately 25 to 35%. A gas CHP shows an example of a so-called micro-combined heat and power plant (micro CHP). Such a plant in the low heat capacity section is referred to as "power generating heating”. It reaches a GHG reduction of about 39% compared to a condensing boiler through a credit for the electricity produced in addition to the heat .
Life cycle costs of a heating system depend on many constraints such as energy prices, taxes, subsidies, rate of interest and inflation, depreciation period, size, quality, energy efficiency, features, market maturity and comfort of the product, individual mounting situation, regional availability of resources, climate conditions, wage costs and so on. Principally they consist of three components:
The capital costs are determined by the investment costs, but also e.g. by the assumed rate of interest and depreciation period or by governmental taxes or support. Operational costs are expenditures for operation and maintenance (O&M), e.g. auxiliary materials, parts subject to wear and wages for staff and insurances. Consumption costs are determined by the energy consumption and thus they highly depend on the energy efficiency of the heating system and on the kind, availability and - in the end - prices of the used energy. As a rule heating technologies of high efficiency and / or with integration of renewable energy have higher upfront investment and hence higher capital costs compared to conventional technologies. On the other hand they save energy, i.e. consumption costs over life time and minimize the risk of rising energy prices (see table below).
| Technology | Capital costs | Operational costs | Consumption costs |
|---|---|---|---|
| Low temperature boiler | low | low | high |
| Condensing boiler | low | low | medium |
| CHP (internal combustion) | high | medium to high | low |
| CHP (stirling engine) | high | medium | medium to low |
| CHP (fuel cell) | very high | medium | very low |
| Solar thermal | high | low | very low |
| Heat pump (air source) | medium | low | medium |
| Heat pump (ground source) | high | low | low |
| Pellet boiler | high | low to medium | low |
The following tables give an exemplary overview of specific investment costs of some advanced heating technologies with high-energy efficiency performance (CHP) or with the integration of renewable energy (solar heating systems, heat pumps) in USD2007/kWth for OECD countries. Cardinally smaller systems (e.g. suitable for single family homes) are specifically more expensive than bigger systems (e.g. for multi-family-dwellings). That’s why it might be more reasonable to relate specific investment costs for heating systems not to the installed capacity (in kWth) but to the heated floor area (see for example the cost curves of diverse heating technologies in (BMVBS 2012) for the energetic renovation of residential buildings).
| USD2007/kWel | Large scale | Small scale |
|---|---|---|
| Reciprocating engines | 1,000 – 1,600 | 1,500 – 12,000 |
| Gas and micro-turbines | 1,050 – 2,000 | 2,000 – 2,700 |
| Fuel cells | 5,000 – 11,000 | 8,000 – 28,000 |
| USD2007/kWth | North America | China + India | OECD Pacific | OECD Europe |
|---|---|---|---|---|
| air-to-air | 360 – 625 | 180 – 225 | 400 – 536 | 558 – 1,430 |
| ASHP | ASHP | ASHP | ASHP | ASHP |
| (Air Source Heat Pump) | 475 – 650 | 300 – 400 | 560 – 1,333 | 607 – 3187 |
| Costs inUSD2007/kWth | Europe | Europe | North America | North America | Pacific | Pacific |
|---|---|---|---|---|---|---|
| - | new build | retrofit | new build | retrofit | new build | retrofit |
| Single-family dwellings | 1,140 – 1,340 | 1,530 – 1,730 | 1,200 – 2,100 | 1,530 – 2,100 | 1,100 – 2,140 | 1,300 - 2,200 |
| Multi-family | 950 – 1,050 | 1,140 – 1,340 | 950 – 1,050 | 1,140 – 1,340 | 1,100 – 1,850 | 1,850 – 2,050 |
Both for the renewable energy using technologies Solar Thermal and Heat Pump as for the new CHP technologies Fuel Cell and Micro-Turbines further reduction in investment costs are expected for the future as show in the table below.
| USD2007/kWth | 2030 | 2050 |
|---|---|---|
| Active solar thermal | -50% to -75% | -50% to -75% |
| Heat pumps (Space / water heating) | -20% to -30% | -30% to -40% |
| CHP: Fuel cells | -40% to -55% | -60% to -75% |
| Micro-turbines | -20% to -30% | -30% to -50% |
Two examples for life a cycle cost comparison of heating systems
As described above numerous factors influence the result of a life cycle costs calculation. Therefore for every application in practice an individual calculation tailored to the local conditions is essential. In the following, two exemplary calculations are presented – one without and the other with the assumption of energy price increases over lifetime. Due to the varying assumptions the results of the both cost calculations are very different: In the first example with constant energy prices systems with low invest / capital costs are best. In the second example with increasing energy prices of 3 to 6 % per year the capital-intensive renewable and energy-efficient systems score better. Both examples are from the central European region (Germany with HDD18 of about 3155). The compilations in the following figure show life cycle costs for different conventional and advanced heating systems. The calculation has been done by ASUE, a German working group for the economical use of gas technologies, and bases on the following assumptions:
| All prices including VAT of 19 %, no energy price increases over next 20 years | Base price €/a | Working price | Heating value |
|---|---|---|---|
| Natural gas | 165 | 0.058 €/kWh | - |
| Bio methane | 165 | 0.128 €/kWh | - |
| Liquid gas | 240 | 0.597 €/l | 6.53 kWh/l |
| Fuel oil - Low sulphur | - | 0.809 €/l | 10.081 kWh/l |
| Fuel oil - Standard | - | 0.808 €/l | 10.081 kWh/l |
| Local / block heating | 350 | 0.068 €/kWh | - |
| District heating | 500 | 0.060 €/kWh | - |
| Pellets | - | 0.225 €/kg | 4.9 kWh/kg |
| Electricity - Standard tariff | - | 0.214 €/kWh | - |
| Electricity - HP-tariff | 80 | 0.120 €/kWh | - |
Cost comparison of energiesparen-im-haushalt.de (energy saving in households) 2013
The calculation is made by energiesparen-im-haushalt.de, an independent Internet portal for the net-working of homeowner, manufacturer and energy consultants. It bases on the following assumptions:
| Type of heating system | Geothermal heat pump | Wood pellets | Gas condensing + solar | Gas condensing | Oil condensing |
|---|---|---|---|---|---|
| Investment and capital costs | . | . | . | . | . |
| Heat generator | 9,000 | 9,000 | 3,000 | 3,000 | 4,000 |
| Installation | 1,500 | 1,500 | 2,000 | 1,000 | 1,000 |
| Geothermal probe, storage, collectors, silo / tank | 4,000 | 1,500 | 6,000 | 1,500 | 2,000 |
| Total investment costs | 14,500 € | 12,000 € | 11,000 € | 5,500 € | 7,000 € |
| Annual capital costs | 1,067 €/a | 883 €/a | 809 €/a | 405 €/a | 515 €/a |
| Consumption and operational costs (first year) | . | . | . | . | . |
| Pellet / Gas / Oil | - | 599 €/a (2.1 t) |
499 €/a (5,300 kWh) |
807 €/a (9,900 kWh) |
948 €/a (990 l) |
| Electricity | 728 €/a (2.800 kWh) |
156 €/a (600 kWh) |
130 €/a (500 kWh) |
104 €/a (400 kWh) |
104 €/a (400 kWh) |
| Total consumption costs | 728 €/a | 755 €/a | 629 €/a | 911 €/a | 1,052 €/a |
| Maintenance costs | 100 €/a | 250 €/a | 250 €/a | 170 €/a | 300 €/a |
| Consumption and operational costs (21th year) | . | . | . | . | . |
| Pellet / Gas / Oil | - | 1,081 €/a (2.1 t) |
1,601 €/a (5,300 kWh) |
2,589 €/a (9,900 kWh) |
3,039€/a (990 l) |
| Electricity | 1,932 €/a | 414 €/a | 345 €/a | 276 €/a | 276 €/a |
| Total consumption costs | 1,932 €/a | 1,495 €/a | 1,946 €/a | 2,865 €/a | 3,315 €/a |
| Maintenance costs | 148 €/a | 371 €/a | 371 €/a | 253 €/a | 446 €/a |
| Total life cycle costs (over 20 years) | . | . | . | . | . |
| Cumulated costs within 20 years (€) | 47,840 € | 44,970 € | 44,920 € | 45,360 € | 55,890 € |
The strategic approach for selecting an appropriate heating system should be: Firstly reduce the building’s heating demand through passive design options, secondly choose an energy-efficient heating system with low GHG emissions and if possible with cogeneration of heat and power, thirdly - if possible - supplied or supported by local renewable resources (solar energy, geothermal or ambient heat, sustainable biomass).
Dietmar Schüwer
References
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